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Classification, groups subordinate to groups -- Natural system -- Rules anddifficulties in classification, explained on the theory of descent withmodification -- Classification of varieties -- Descent always used inclassification -- Analogical or adaptive characters -- Affinities, general,complex and radiating -- Extinction separates and defines groups --Morphology, between members of the same class, between parts of the sameindividual -- Embryology, laws of, explained by variations not superveningat an early age, and being inherited at a corresponding age -- Rudimentaryorgans; their origin explained -- Summary.

CLASSIFICATION.

>From the most remote period in the history of the world organic beings havebeen found to resemble each other in descending degrees, so that they canbe classed in groups under groups. This classification is not arbitrarylike the grouping of the stars in constellations. The existence of groupswould have been of simple significance, if one group had been exclusivelyfitted to inhabit the land, and another the water; one to feed on flesh,another on vegetable matter, and so on; but the case is widely different,for it is notorious how commonly members of even the same subgroup havedifferent habits. In the second and fourth chapters, on Variation and onNatural Selection, I have attempted to show that within each country it isthe widely ranging, the much diffused and common, that is the dominantspecies, belonging to the larger genera in each class, which vary most. The varieties, or incipient species, thus produced, ultimately becomeconverted into new and distinct species; and these, on the principle ofinheritance, tend to produce other new and dominant species. Consequentlythe groups which are now large, and which generally include many dominantspecies, tend to go on increasing in size. I further attempted to showthat from the varying descendants of each species trying to occupy as manyand as different places as possible in the economy of nature, theyconstantly tend to diverge in character. This latter conclusion issupported by observing the great diversity of forms, which, in any smallarea, come into the closest competition, and by certain facts innaturalisation.

I attempted also to show that there is a steady tendency in the forms whichare increasing in number and diverging in character, to supplant andexterminate the preceding, less divergent and less improved forms. Irequest the reader to turn to the diagram illustrating the action, asformerly explained, of these several principles; and he will see that theinevitable result is, that the modified descendants proceeding from oneprogenitor become broken up into groups subordinate to groups. In thediagram each letter on the uppermost line may represent a genus includingseveral species; and the whole of the genera along this upper line formtogether one class, for all are descended from one ancient parent, and,consequently, have inherited something in common. But the three genera onthe left hand have, on this same principle, much in common, and form asubfamily, distinct from that containing the next two genera on the righthand, which diverged from a common parent at the fifth stage of descent. These five genera have also much in common, though less than when groupedin subfamilies; and they form a family distinct from that containing thethree genera still further to the right hand, which diverged at an earlierperiod. And all these genera, descended from (A), form an order distinctfrom the genera descended from (I). So that we here have many speciesdescended from a single progenitor grouped into genera; and the genera intosubfamilies, families and orders, all under one great class. The grandfact of the natural subordination of organic beings in groups under groups,which, from its familiarity, does not always sufficiently strike us, is inmy judgment thus explained. No doubt organic beings, like all otherobjects, can be classed in many ways, either artificially by singlecharacters, or more naturally by a number of characters. We know, forinstance, that minerals and the elemental substances can be thus arranged.In this case there is of course no relation to genealogical succession, andno cause can at present be assigned for their falling into groups. Butwith organic beings the case is different, and the view above given accordswith their natural arrangement in group under group; and no otherexplanation has ever been attempted.

Naturalists, as we have seen, try to arrange the species, genera andfamilies in each class, on what is called the Natural System. But what ismeant by this system? Some authors look at it merely as a scheme forarranging together those living objects which are most alike, and forseparating those which are most unlike; or as an artificial method ofenunciating, as briefly as possible, general propositions--that is, by onesentence to give the characters common, for instance, to all mammals, byanother those common to all carnivora, by another those common to thedog-genus, and then, by adding a single sentence, a full description isgiven of each kind of dog. The ingenuity and utility of this system areindisputable. But many naturalists think that something more is meant bythe Natural System; they believe that it reveals the plan of the Creator;but unless it be specified whether order in time or space, or both, or whatelse is meant by the plan of the Creator, it seems to me that nothing isthus added to our knowledge. Expressions such as that famous one byLinnaeus, which we often meet with in a more or less concealed form,namely, that the characters do not make the genus, but that the genus givesthe characters, seem to imply that some deeper bond is included in ourclassifications than mere resemblance. I believe that this is the case,and that community of descent--the one known cause of close similarity inorganic beings--is the bond, which, though observed by various degrees ofmodification, is partially revealed to us by our classifications.

Let us now consider the rules followed in classification, and thedifficulties which are encountered on the view that classification eithergives some unknown plan of creation, or is simply a scheme for enunciatinggeneral propositions and of placing together the forms most like eachother. It might have been thought (and was in ancient times thought) thatthose parts of the structure which determined the habits of life, and thegeneral place of each being in the economy of nature, would be of very highimportance in classification. Nothing can be more false. No one regardsthe external similarity of a mouse to a shrew, of a dugong to a whale, of awhale to a fish, as of any importance. These resemblances, though sointimately connected with the whole life of the being, are ranked as merely"adaptive or analogical characters;" but to the consideration of theseresemblances we shall recur. It may even be given as a general rule, thatthe less any part of the organisation is concerned with special habits, themore important it becomes for classification. As an instance: Owen, inspeaking of the dugong, says, "The generative organs, being those which aremost remotely related to the habits and food of an animal, I have alwaysregarded as affording very clear indications of its true affinities. Weare least likely in the modifications of these organs to mistake a merelyadaptive for an essential character." With plants how remarkable it isthat the organs of vegetation, on which their nutrition and life depend,are of little signification; whereas the organs of reproduction, with theirproduct the seed and embryo, are of paramount importance! So again, informerly discussing certain morphological characters which are notfunctionally important, we have seen that they are often of the highestservice in classification. This depends on their constancy throughout manyallied groups; and their constancy chiefly depends on any slight deviationsnot having been preserved and accumulated by natural selection, which actsonly on serviceable characters.

That the mere physiological importance of an organ does not determine itsclassificatory value, is almost proved by the fact, that in allied groups,in which the same organ, as we have every reason to suppose, has nearly thesame physiological value, its classificatory value is widely different. Nonaturalist can have worked at any group without being struck with thisfact; and it has been fully acknowledged in the writings of almost everyauthor. It will suffice to quote the highest authority, Robert Brown, who,in speaking of certain organs in the Proteaceae, says their genericimportance, "like that of all their parts, not only in this, but, as Iapprehend in every natural family, is very unequal, and in some cases seemsto be entirely lost." Again, in another work he says, the genera of theConnaraceae "differ in having one or more ovaria, in the existence orabsence of albumen, in the imbricate or valvular aestivation. Any one ofthese characters singly is frequently of more than generic importance,though here even, when all taken together, they appear insufficient toseparate Cnestis from Connarus." To give an example among insects: in onegreat division of the Hymenoptera, the antennae, as Westwood has remarked,are most constant in structure; in another division they differ much, andthe differences are of quite subordinate value in classification; yet noone will say that the antennae in these two divisions of the same order areof unequal physiological importance. Any number of instances could begiven of the varying importance for classification of the same importantorgan within the same group of beings.

Again, no one will say that rudimentary or atrophied organs are of highphysiological or vital importance; yet, undoubtedly, organs in thiscondition are often of much value in classification. No one will disputethat the rudimentary teeth in the upper jaws of young ruminants, andcertain rudimentary bones of the leg, are highly serviceable in exhibitingthe close affinity between Ruminants and Pachyderms. Robert Brown hasstrongly insisted on the fact that the position of the rudimentary floretsis of the highest importance in the classification of the Grasses.

Numerous instances could be given of characters derived from parts whichmust be considered of very trifling physiological importance, but which areuniversally admitted as highly serviceable in the definition of wholegroups. For instance, whether or not there is an open passage from thenostrils to the mouth, the only character, according to Owen, whichabsolutely distinguishes fishes and reptiles--the inflection of the angleof the lower jaw in Marsupials--the manner in which the wings of insectsare folded--mere colour in certain Algae--mere pubescence on parts of theflower in grasses--the nature of the dermal covering, as hair or feathers,in the Vertebrata. If the Ornithorhynchus had been covered with feathersinstead of hair, this external and trifling character would have beenconsidered by naturalists as an important aid in determining the degree ofaffinity of this strange creature to birds.

The importance, for classification, of trifling characters, mainly dependson their being correlated with many other characters of more or lessimportance. The value indeed of an aggregate of characters is very evidentin natural history. Hence, as has often been remarked, a species maydepart from its allies in several characters, both of high physiologicalimportance, and of almost universal prevalence, and yet leave us in nodoubt where it should be ranked. Hence, also, it has been found that aclassification founded on any single character, however important that maybe, has always failed; for no part of the organisation is invariablyconstant. The importance of an aggregate of characters, even when none areimportant, alone explains the aphorism enunciated by Linnaeus, namely, thatthe characters do not give the genus, but the genus gives the character;for this seems founded on the appreciation of many trifling points ofresemblance, too slight to be defined. Certain plants, belonging to theMalpighiaceae, bear perfect and degraded flowers; in the latter, as A. deJussieu has remarked, "The greater number of the characters proper to thespecies, to the genus, to the family, to the class, disappear, and thuslaugh at our classification." When Aspicarpa produced in France, duringseveral years, only these degraded flowers, departing so wonderfully in anumber of the most important points of structure from the proper type ofthe order, yet M. Richard sagaciously saw, as Jussieu observes, that thisgenus should still be retained among the Malpighiaceae. This case wellillustrates the spirit of our classifications.

Practically, when naturalists are at work, they do not trouble themselvesabout the physiological value of the characters which they use in defininga group or in allocating any particular species. If they find a characternearly uniform, and common to a great number of forms, and not common toothers, they use it as one of high value; if common to some lesser number,they use it as of subordinate value. This principle has been broadlyconfessed by some naturalists to be the true one; and by none more clearlythan by that excellent botanist, Aug. St. Hilaire. If several triflingcharacters are always found in combination, though no apparent bond ofconnexion can be discovered between them, especial value is set on them. As in most groups of animals, important organs, such as those forpropelling the blood, or for aerating it, or those for propagating therace, are found nearly uniform, they are considered as highly serviceablein classification; but in some groups all these, the most important vitalorgans, are found to offer characters of quite subordinate value. Thus, asFritz Muller has lately remarked, in the same group of crustaceans,Cypridina is furnished with a heart, while in two closely allied genera,namely Cypris and Cytherea, there is no such organ; one species ofCypridina has well-developed branchiae, while another species is destituteof them.

We can see why characters derived from the embryo should be of equalimportance with those derived from the adult, for a natural classificationof course includes all ages. But it is by no means obvious, on theordinary view, why the structure of the embryo should be more important forthis purpose than that of the adult, which alone plays its full part in theeconomy of nature. Yet it has been strongly urged by those greatnaturalists, Milne Edwards and Agassiz, that embryological characters arethe most important of all; and this doctrine has very generally beenadmitted as true. Nevertheless, their importance has sometimes beenexaggerated, owing to the adaptive characters of larvae not having beenexcluded; in order to show this, Fritz Muller arranged, by the aid of suchcharacters alone, the great class of crustaceans, and the arrangement didnot prove a natural one. But there can be no doubt that embryonic,excluding larval characters, are of the highest value for classification,not only with animals but with plants. Thus the main divisions offlowering plants are founded on differences in the embryo--on the numberand position of the cotyledons, and on the mode of development of theplumule and radicle. We shall immediately see why these characters possessso high a value in classification, namely, from the natural system beinggenealogical in its arrangement.

Our classifications are often plainly influenced by chains of affinities. Nothing can be easier than to define a number of characters common to allbirds; but with crustaceans, any such definition has hitherto been foundimpossible. There are crustaceans at the opposite ends of the series,which have hardly a character in common; yet the species at both ends, frombeing plainly allied to others, and these to others, and so onwards, can berecognised as unequivocally belonging to this, and to no other class of theArticulata.

Geographical distribution has often been used, though perhaps not quitelogically, in classification, more especially in very large groups ofclosely allied forms. Temminck insists on the utility or even necessity ofthis practice in certain groups of birds; and it has been followed byseveral entomologists and botanists.

Finally, with respect to the comparative value of the various groups ofspecies, such as orders, suborders, families, subfamilies, and genera, theyseem to be, at least at present, almost arbitrary. Several of the bestbotanists, such as Mr. Bentham and others, have strongly insisted on theirarbitrary value. Instances could be given among plants and insects, of agroup first ranked by practised naturalists as only a genus, and thenraised to the rank of a subfamily or family; and this has been done, notbecause further research has detected important structural differences, atfirst overlooked, but because numerous allied species, with slightlydifferent grades of difference, have been subsequently discovered.

All the foregoing rules and aids and difficulties in classification may beexplained, if I do not greatly deceive myself, on the view that the naturalsystem is founded on descent with modification--that the characters whichnaturalists consider as showing true affinity between any two or morespecies, are those which have been inherited from a common parent, all trueclassification being genealogical--that community of descent is the hiddenbond which naturalists have been unconsciously seeking, and not someunknown plan of creation, or the enunciation of general propositions, andthe mere putting together and separating objects more or less alike.

But I must explain my meaning more fully. I believe that the ARRANGEMENTof the groups within each class, in due subordination and relation to eachother, must be strictly genealogical in order to be natural; but that theAMOUNT of difference in the several branches or groups, though allied inthe same degree in blood to their common progenitor, may differ greatly,being due to the different degrees of modification which they haveundergone; and this is expressed by the forms being ranked under differentgenera, families, sections or orders. The reader will best understand whatis meant, if he will take the trouble to refer to the diagram in the fourthchapter. We will suppose the letters A to L to represent allied generaexisting during the Silurian epoch, and descended from some still earlierform. In three of these genera (A, F, and I) a species has transmittedmodified descendants to the present day, represented by the fifteen genera(a14 to z14) on the uppermost horizontal line. Now, all these modifieddescendants from a single species are related in blood or descent in thesame degree. They may metaphorically be called cousins to the samemillionth degree, yet they differ widely and in different degrees from eachother. The forms descended from A, now broken up into two or threefamilies, constitute a distinct order from those descended from I, alsobroken up into two families. Nor can the existing species descended from Abe ranked in the same genus with the parent A, or those from I with parentI. But the existing genus F14 may be supposed to have been but slightlymodified, and it will then rank with the parent genus F; just as some fewstill living organisms belong to Silurian genera. So that the comparativevalue of the differences between these organic beings, which are allrelated to each other in the same degree in blood, has come to be widelydifferent. Nevertheless, their genealogical ARRANGEMENT remains strictlytrue, not only at the present time, but at each successive period ofdescent. All the modified descendants from A will have inherited somethingin common from their common parent, as will all the descendants from I; sowill it be with each subordinate branch of descendants at each successivestage. If, however, we suppose any descendant of A or of I to have becomeso much modified as to have lost all traces of its parentage in this case,its place in the natural system will be lost, as seems to have occurredwith some few existing organisms. All the descendants of the genus F,along its whole line of descent, are supposed to have been but littlemodified, and they form a single genus. But this genus, though muchisolated, will still occupy its proper intermediate position. Therepresentation of the groups as here given in the diagram on a flatsurface, is much too simple. The branches ought to have diverged in alldirections. If the names of the groups had been simply written down in alinear series the representation would have been still less natural; and itis notoriously not possible to represent in a series, on a flat surface,the affinities which we discover in nature among the beings of the samegroup. Thus, the natural system is genealogical in its arrangement, like apedigree. But the amount of modification which the different groups haveundergone has to be expressed by ranking them under different so-calledgenera, subfamilies, families, sections, orders, and classes.

It may be worth while to illustrate this view of classification, by takingthe case of languages. If we possessed a perfect pedigree of mankind, agenealogical arrangement of the races of man would afford the bestclassification of the various languages now spoken throughout the world;and if all extinct languages, and all intermediate and slowly changingdialects, were to be included, such an arrangement would be the onlypossible one. Yet it might be that some ancient languages had altered verylittle and had given rise to few new languages, whilst others had alteredmuch owing to the spreading, isolation and state of civilisation of theseveral co-descended races, and had thus given rise to many new dialectsand languages. The various degrees of difference between the languages ofthe same stock would have to be expressed by groups subordinate to groups;but the proper or even the only possible arrangement would still begenealogical; and this would be strictly natural, as it would connecttogether all languages, extinct and recent, by the closest affinities, andwould give the filiation and origin of each tongue.

In confirmation of this view, let us glance at the classification ofvarieties, which are known or believed to be descended from a singlespecies. These are grouped under the species, with the subvarieties underthe varieties; and in some cases, as with the domestic pigeon, with severalother grades of difference. Nearly the same rules are followed as inclassifying species. Authors have insisted on the necessity of arrangingvarieties on a natural instead of an artificial system; we are cautioned,for instance, not to class two varieties of the pine-apple together, merelybecause their fruit, though the most important part, happens to be nearlyidentical; no one puts the Swedish and common turnip together, though theesculent and thickened stems are so similar. Whatever part is found to bemost constant, is used in classing varieties: thus the great agriculturistMarshall says the horns are very useful for this purpose with cattle,because they are less variable than the shape or colour of the body, etc.;whereas with sheep the horns are much less serviceable, because lessconstant. In classing varieties, I apprehend that if we had a realpedigree, a genealogical classification would be universally preferred; andit has been attempted in some cases. For we might feel sure, whether therehad been more or less modification, that the principle of inheritance wouldkeep the forms together which were allied in the greatest number of points. In tumbler pigeons, though some of the subvarieties differ in the importantcharacter of the length of the beak, yet all are kept together from havingthe common habit of tumbling; but the short-faced breed has nearly or quitelost this habit; nevertheless, without any thought on the subject, thesetumblers are kept in the same group, because allied in blood and alike insome other respects.

With species in a state of nature, every naturalist has in fact broughtdescent into his classification; for he includes in his lowest grade, thatof species, the two sexes; and how enormously these sometimes differ in themost important characters is known to every naturalist: scarcely a singlefact can be predicated in common of the adult males and hermaphrodites ofcertain cirripedes, and yet no one dreams of separating them. As soon asthe three Orchidean forms, Monachanthus, Myanthus, and Catasetum, which hadpreviously been ranked as three distinct genera, were known to be sometimesproduced on the same plant, they were immediately considered as varieties;and now I have been able to show that they are the male, female, andhermaphrodite forms of the same species. The naturalist includes as onespecies the various larval stages of the same individual, however much theymay differ from each other and from the adult; as well as the so-calledalternate generations of Steenstrup, which can only in a technical sense beconsidered as the same individual. He includes monsters and varieties, notfrom their partial resemblance to the parent-form, but because they aredescended from it.

As descent has universally been used in classing together the individualsof the same species, though the males and females and larvae are sometimesextremely different; and as it has been used in classing varieties whichhave undergone a certain, and sometimes a considerable amount ofmodification, may not this same element of descent have been unconsciouslyused in grouping species under genera, and genera under higher groups, allunder the so-called natural system? I believe it has been unconsciouslyused; and thus only can I understand the several rules and guides whichhave been followed by our best systematists. As we have no writtenpedigrees, we are forced to trace community of descent by resemblances ofany kind. Therefore, we choose those characters which are the least likelyto have been modified, in relation to the conditions of life to which eachspecies has been recently exposed. Rudimentary structures on this view areas good as, or even sometimes better than other parts of the organisation. We care not how trifling a character may be--let it be the mere inflectionof the angle of the jaw, the manner in which an insect's wing is folded,whether the skin be covered by hair or feathers--if it prevail throughoutmany and different species, especially those having very different habitsof life, it assumes high value; for we can account for its presence in somany forms with such different habits, only by inheritance from a commonparent. We may err in this respect in regard to single points ofstructure, but when several characters, let them be ever so trifling,concur throughout a large group of beings having different habits, we mayfeel almost sure, on the theory of descent, that these characters have beeninherited from a common ancestor; and we know that such aggregatedcharacters have especial value in classification.

We can understand why a species or a group of species may depart from itsallies, in several of its most important characteristics, and yet be safelyclassed with them. This may be safely done, and is often done, as long asa sufficient number of characters, let them be ever so unimportant, betraysthe hidden bond of community of descent. Let two forms have not a singlecharacter in common, yet, if these extreme forms are connected together bya chain of intermediate groups, we may at once infer their community ofdescent, and we put them all into the same class. As we find organs ofhigh physiological importance--those which serve to preserve life under themost diverse conditions of existence--are generally the most constant, weattach especial value to them; but if these same organs, in another groupor section of a group, are found to differ much, we at once value them lessin our classification. We shall presently see why embryological charactersare of such high classificatory importance. Geographical distribution maysometimes be brought usefully into play in classing large genera, becauseall the species of the same genus, inhabiting any distinct and isolatedregion, are in all probability descended from the same parents.

ANALOGICAL RESEMBLANCES.

We can understand, on the above views, the very important distinctionbetween real affinities and analogical or adaptive resemblances. Lamarckfirst called attention to this subject, and he has been ably followed byMacleay and others. The resemblance in the shape of the body and in thefin-like anterior limbs between dugongs and whales, and between these twoorders of mammals and fishes, are analogical. So is the resemblancebetween a mouse and a shrew-mouse (Sorex), which belong to differentorders; and the still closer resemblance, insisted on by Mr. Mivart,between the mouse and a small marsupial animal (Antechinus) of Australia. These latter resemblances may be accounted for, as it seems to me, byadaptation for similarly active movements through thickets and herbage,together with concealment from enemies.

Among insects there are innumerable instances; thus Linnaeus, misled byexternal appearances, actually classed an homopterous insect as a moth. Wesee something of the same kind even with our domestic varieties, as in thestrikingly similar shape of the body in the improved breeds of the Chineseand common pig, which are descended from distinct species; and in thesimilarly thickened stems of the common and specifically distinct Swedishturnip. The resemblance between the greyhound and race-horse is hardlymore fanciful than the analogies which have been drawn by some authorsbetween widely different animals.

On the view of characters being of real importance for classification, onlyin so far as they reveal descent, we can clearly understand why analogicalor adaptive characters, although of the utmost importance to the welfare ofthe being, are almost valueless to the systematist. For animals, belongingto two most distinct lines of descent, may have become adapted to similarconditions, and thus have assumed a close external resemblance; but suchresemblances will not reveal--will rather tend to conceal theirblood-relationship. We can thus also understand the apparent paradox, thatthe very same characters are analogical when one group is compared withanother, but give true affinities when the members of the same group arecompared together: thus the shape of the body and fin-like limbs are onlyanalogical when whales are compared with fishes, being adaptations in bothclasses for swimming through the water; but between the the severalmembers of the whale family, the shape of the body and the fin-like limbsoffer characters exhibiting true affinity; for as these parts are so nearlysimilar throughout the whole family, we cannot doubt that they have beeninherited from a common ancestor. So it is with fishes.

Numerous cases could be given of striking resemblances in quite distinctbeings between single parts or organs, which have been adapted for the samefunctions. A good instance is afforded by the close resemblance of thejaws of the dog and Tasmanian wolf or Thylacinus--animals which are widelysundered in the natural system. But this resemblance is confined togeneral appearance, as in the prominence of the canines, and in the cuttingshape of the molar teeth. For the teeth really differ much: thus the doghas on each side of the upper jaw four pre-molars and only two molars;while the Thylacinus has three pre-molars and four molars. The molars alsodiffer much in the two animals in relative size and structure. The adultdentition is preceded by a widely different milk dentition. Any one may,of course, deny that the teeth in either case have been adapted for tearingflesh, through the natural selection of successive variations; but if thisbe admitted in the one case, it is unintelligible to me that it should bedenied in the other. I am glad to find that so high an authority asProfessor Flower has come to this same conclusion.

The extraordinary cases given in a former chapter, of widely differentfishes possessing electric organs--of widely different insects possessingluminous organs--and of orchids and asclepiads having pollen-masses withviscid discs, come under this same head of analogical resemblances. Butthese cases are so wonderful that they were introduced as difficulties orobjections to our theory. In all such cases some fundamental difference inthe growth or development of the parts, and generally in their maturedstructure, can be detected. The end gained is the same, but the means,though appearing superficially to be the same, are essentially different. The principle formerly alluded to under the term of ANALOGICAL VARIATIONhas probably in these cases often come into play; that is, the members ofthe same class, although only distantly allied, have inherited so much incommon in their constitution, that they are apt to vary under similarexciting causes in a similar manner; and this would obviously aid in theacquirement through natural selection of parts or organs, strikingly likeeach other, independently of their direct inheritance from a commonprogenitor.

As species belonging to distinct classes have often been adapted bysuccessive slight modifications to live under nearly similar circumstances--to inhabit, for instance, the three elements of land, air and water--wecan perhaps understand how it is that a numerical parallelism has sometimesbeen observed between the subgroups of distinct classes. A naturalist,struck with a parallelism of this nature, by arbitrarily raising or sinkingthe value of the groups in several classes (and all our experience showsthat their valuation is as yet arbitrary), could easily extend theparallelism over a wide range; and thus the septenary, quinary, quaternaryand ternary classifications have probably arisen.

There is another and curious class of cases in which close externalresemblance does not depend on adaptation to similar habits of life, buthas been gained for the sake of protection. I allude to the wonderfulmanner in which certain butterflies imitate, as first described by Mr.Bates, other and quite distinct species. This excellent observer has shownthat in some districts of South America, where, for instance, an Ithomiaabounds in gaudy swarms, another butterfly, namely, a Leptalis, is oftenfound mingled in the same flock; and the latter so closely resembles theIthomia in every shade and stripe of colour, and even in the shape of itswings, that Mr. Bates, with his eyes sharpened by collecting during elevenyears, was, though always on his guard, continually deceived. When themockers and the mocked are caught and compared, they are found to be verydifferent in essential structure, and to belong not only to distinctgenera, but often to distinct families. Had this mimicry occurred in onlyone or two instances, it might have been passed over as a strangecoincidence. But, if we proceed from a district where one Leptalisimitates an Ithomia, another mocking and mocked species, belonging to thesame two genera, equally close in their resemblance, may be found. Altogether no less than ten genera are enumerated, which include speciesthat imitate other butterflies. The mockers and mocked always inhabit thesame region; we never find an imitator living remote from the form which itimitates. The mockers are almost invariably rare insects; the mocked inalmost every case abounds in swarms. In the same district in which aspecies of Leptalis closely imitates an Ithomia, there are sometimes otherLepidoptera mimicking the same Ithomia: so that in the same place, speciesof three genera of butterflies and even a moth are found all closelyresembling a butterfly belonging to a fourth genus. It deserves especialnotice that many of the mimicking forms of the Leptalis, as well as of themimicked forms, can be shown by a graduated series to be merely varietiesof the same species; while others are undoubtedly distinct species. Butwhy, it may be asked, are certain forms treated as the mimicked and othersas the mimickers? Mr. Bates satisfactorily answers this question byshowing that the form which is imitated keeps the usual dress of the groupto which it belongs, while the counterfeiters have changed their dress anddo not resemble their nearest allies.

We are next led to enquire what reason can be assigned for certainbutterflies and moths so often assuming the dress of another and quitedistinct form; why, to the perplexity of naturalists, has naturecondescended to the tricks of the stage? Mr. Bates has, no doubt, hit onthe true explanation. The mocked forms, which always abound in numbers,must habitually escape destruction to a large extent, otherwise they couldnot exist in such swarms; and a large amount of evidence has now beencollected, showing that they are distasteful to birds and other insect-devouring animals. The mocking forms, on the other hand, that inhabit thesame district, are comparatively rare, and belong to rare groups; hence,they must suffer habitually from some danger, for otherwise, from thenumber of eggs laid by all butterflies, they would in three or fourgenerations swarm over the whole country. Now if a member of one of thesepersecuted and rare groups were to assume a dress so like that of a well-protected species that it continually deceived the practised eyes of anentomologist, it would often deceive predaceous birds and insects, and thusoften escape destruction. Mr. Bates may almost be said to have actuallywitnessed the process by which the mimickers have come so closely toresemble the mimicked; for he found that some of the forms of Leptaliswhich mimic so many other butterflies, varied in an extreme degree. In onedistrict several varieties occurred, and of these one alone resembled, to acertain extent, the common Ithomia of the same district. In anotherdistrict there were two or three varieties, one of which was much commonerthan the others, and this closely mocked another form of Ithomia. Fromfacts of this nature, Mr. Bates concludes that the Leptalis first varies;and when a variety happens to resemble in some degree any common butterflyinhabiting the same district, this variety, from its resemblance to aflourishing and little persecuted kind, has a better chance of escapingdestruction from predaceous birds and insects, and is consequently oftenerpreserved; "the less perfect degrees of resemblance being generation aftergeneration eliminated, and only the others left to propagate their kind." So that here we have an excellent illustration of natural selection.

Messrs. Wallace and Trimen have likewise described several equally strikingcases of imitation in the Lepidoptera of the Malay Archipelago and Africa,and with some other insects. Mr. Wallace has also detected one such casewith birds, but we have none with the larger quadrupeds. The much greaterfrequency of imitation with insects than with other animals, is probablythe consequence of their small size; insects cannot defend themselves,excepting indeed the kinds furnished with a sting, and I have never heardof an instance of such kinds mocking other insects, though they are mocked;insects cannot easily escape by flight from the larger animals which preyon them; therefore, speaking metaphorically, they are reduced, like mostweak creatures, to trickery and dissimulation.

It should be observed that the process of imitation probably nevercommenced between forms widely dissimilar in colour. But, starting withspecies already somewhat like each other, the closest resemblance, ifbeneficial, could readily be gained by the above means, and if the imitatedform was subsequently and gradually modified through any agency, theimitating form would be led along the same track, and thus be altered toalmost any extent, so that it might ultimately assume an appearance orcolouring wholly unlike that of the other members of the family to which itbelonged. There is, however, some difficulty on this head, for it isnecessary to suppose in some cases that ancient members belonging toseveral distinct groups, before they had diverged to their present extent,accidentally resembled a member of another and protected group in asufficient degree to afford some slight protection, this having given thebasis for the subsequent acquisition of the most perfect resemblance.

ON THE NATURE OF THE AFFINITIES CONNECTING ORGANIC BEINGS.

As the modified descendants of dominant species, belonging to the largergenera, tend to inherit the advantages which made the groups to which theybelong large and their parents dominant, they are almost sure to spreadwidely, and to seize on more and more places in the economy of nature. Thelarger and more dominant groups within each class thus tend to go onincreasing in size, and they consequently supplant many smaller and feeblergroups. Thus, we can account for the fact that all organisms, recent andextinct, are included under a few great orders and under still fewerclasses. As showing how few the higher groups are in number, and howwidely they are spread throughout the world, the fact is striking that thediscovery of Australia has not added an insect belonging to a new class,and that in the vegetable kingdom, as I learn from Dr. Hooker, it has addedonly two or three families of small size.

In the chapter on geological succession I attempted to show, on theprinciple of each group having generally diverged much in character duringthe long-continued process of modification, how it is that the more ancientforms of life often present characters in some degree intermediate betweenexisting groups. As some few of the old and intermediate forms havingtransmitted to the present day descendants but little modified, theseconstitute our so-called osculant or aberrant groups. The more aberrantany form is, the greater must be the number of connecting forms which havebeen exterminated and utterly lost. And we have evidence of aberrantgroups having suffered severely from extinction, for they are almost alwaysrepresented by extremely few species; and such species as do occur aregenerally very distinct from each other, which again implies extinction. The genera Ornithorhynchus and Lepidosiren, for example, would not havebeen less aberrant had each been represented by a dozen species, instead ofas at present by a single one, or by two or three. We can, I think,account for this fact only by looking at aberrant groups as forms whichhave been conquered by more successful competitors, with a few membersstill preserved under unusually favourable conditions.

Mr. Waterhouse has remarked that when a member belonging to one group ofanimals exhibits an affinity to a quite distinct group, this affinity inmost cases is general and not special: thus, according to Mr. Waterhouse,of all Rodents, the bizcacha is most nearly related to Marsupials; but inthe points in which it approaches this order, its relations are general,that is, not to any one Marsupial species more than to another. As thesepoints of affinity are believed to be real and not merely adaptive, theymust be due in accordance with our view to inheritance from a commonprogenitor. Therefore, we must suppose either that all Rodents, includingthe bizcacha, branched off from some ancient Marsupial, which willnaturally have been more or less intermediate in character with respect toall existing Marsupials; or that both Rodents and Marsupials branched offfrom a common progenitor, and that both groups have since undergone muchmodification in divergent directions. On either view we must suppose thatthe bizcacha has retained, by inheritance, more of the character of itsancient progenitor than have other Rodents; and therefore it will not bespecially related to any one existing Marsupial, but indirectly to all ornearly all Marsupials, from having partially retained the character oftheir common progenitor, or of some early member of the group. On theother hand, of all Marsupials, as Mr. Waterhouse has remarked, thePhascolomys resembles most nearly, not any one species, but the generalorder of Rodents. In this case, however, it may be strongly suspected thatthe resemblance is only analogical, owing to the Phascolomys having becomeadapted to habits like those of a Rodent. The elder De Candolle has madenearly similar observations on the general nature of the affinities ofdistinct families of plants.

On the principle of the multiplication and gradual divergence in characterof the species descended from a common progenitor, together with theirretention by inheritance of some characters in common, we can understandthe excessively complex and radiating affinities by which all the membersof the same family or higher group are connected together. For the commonprogenitor of a whole family, now broken up by extinction into distinctgroups and subgroups, will have transmitted some of its characters,modified in various ways and degrees, to all the species; and they willconsequently be related to each other by circuitous lines of affinity ofvarious lengths (as may be seen in the diagram so often referred to),mounting up through many predecessors. As it is difficult to show theblood-relationship between the numerous kindred of any ancient and noblefamily, even by the aid of a genealogical tree, and almost impossible to doso without this aid, we can understand the extraordinary difficulty whichnaturalists have experienced in describing, without the aid of a diagram,the various affinities which they perceive between the many living andextinct members of the same great natural class.

Extinction, as we have seen in the fourth chapter, has played an importantpart in defining and widening the intervals between the several groups ineach class. We may thus account for the distinctness of whole classes fromeach other--for instance, of birds from all other vertebrate animals--bythe belief that many ancient forms of life have been utterly lost, throughwhich the early progenitors of birds were formerly connected with the earlyprogenitors of the other and at that time less differentiated vertebrateclasses. There has been much less extinction of the forms of life whichonce connected fishes with Batrachians. There has been still less withinsome whole classes, for instance the Crustacea, for here the mostwonderfully diverse forms are still linked together by a long and onlypartially broken chain of affinities. Extinction has only defined thegroups: it has by no means made them; for if every form which has everlived on this earth were suddenly to reappear, though it would be quiteimpossible to give definitions by which each group could be distinguished,still a natural classification, or at least a natural arrangement, would bepossible. We shall see this by turning to the diagram: the letters, A toL, may represent eleven Silurian genera, some of which have produced largegroups of modified descendants, with every link in each branch andsub-branch still alive; and the links not greater than those betweenexisting varieties. In this case it would be quite impossible to givedefinitions by which the several members of the several groups could bedistinguished from their more immediate parents and descendants. Yet thearrangement in the diagram would still hold good and would be natural; for,on the principle of inheritance, all the forms descended, for instance fromA, would have something in common. In a tree we can distinguish this orthat branch, though at the actual fork the two unite and blend together. We could not, as I have said, define the several groups; but we could pickout types, or forms, representing most of the characters of each group,whether large or small, and thus give a general idea of the value of thedifferences between them. This is what we should be driven to, if we wereever to succeed in collecting all the forms in any one class which havelived throughout all time and space. Assuredly we shall never succeed inmaking so perfect a collection: nevertheless, in certain classes, we aretending toward this end; and Milne Edwards has lately insisted, in an ablepaper, on the high importance of looking to types, whether or not we canseparate and define the groups to which such types belong.

Finally, we have seen that natural selection, which follows from thestruggle for existence, and which almost inevitably leads to extinction anddivergence of character in the descendants from any one parent-species,explains that great and universal feature in the affinities of all organicbeings, namely, their subordination in group under group. We use theelement of descent in classing the individuals of both sexes and of allages under one species, although they may have but few characters incommon; we use descent in classing acknowledged varieties, howeverdifferent they may be from their parents; and I believe that this elementof descent is the hidden bond of connexion which naturalists have soughtunder the term of the Natural System. On this idea of the natural systembeing, in so far as it has been perfected, genealogical in its arrangement,with the grades of difference expressed by the terms genera, families,orders, etc., we can understand the rules which we are compelled to followin our classification. We can understand why we value certain resemblancesfar more than others; why we use rudimentary and useless organs, or othersof trifling physiological importance; why, in finding the relations betweenone group and another, we summarily reject analogical or adaptivecharacters, and yet use these same characters within the limits of the samegroup. We can clearly see how it is that all living and extinct forms canbe grouped together within a few great classes; and how the several membersof each class are connected together by the most complex and radiatinglines of affinities. We shall never, probably, disentangle theinextricable web of the affinities between the members of any one class;but when we have a distinct object in view, and do not look to some unknownplan of creation, we may hope to make sure but slow progress.

Professor Haeckel in his "Generelle Morphologie" and in another works, hasrecently brought his great knowledge and abilities to bear on what he callsphylogeny, or the lines of descent of all organic beings. In drawing upthe several series he trusts chiefly to embryological characters, butreceives aid from homologous and rudimentary organs, as well as from thesuccessive periods at which the various forms of life are believed to havefirst appeared in our geological formations. He has thus boldly made agreat beginning, and shows us how classification will in the future betreated.

MORPHOLOGY.

We have seen that the members of the same class, independently of theirhabits of life, resemble each other in the general plan of theirorganisation. This resemblance is often expressed by the term "unity oftype;" or by saying that the several parts and organs in the differentspecies of the class are homologous. The whole subject is included underthe general term of Morphology. This is one of the most interestingdepartments of natural history, and may almost be said to be its very soul. What can be more curious than that the hand of a man, formed for grasping,that of a mole for digging, the leg of the horse, the paddle of theporpoise, and the wing of the bat, should all be constructed on the samepattern, and should include similar bones, in the same relative positions?How curious it is, to give a subordinate though striking instance, that thehind feet of the kangaroo, which are so well fitted for bounding over theopen plains--those of the climbing, leaf-eating koala, equally well fittedfor grasping the branches of trees--those of the ground-dwelling, insect orroot-eating, bandicoots--and those of some other Australian marsupials--should all be constructed on the same extraordinary type, namely with thebones of the second and third digits extremely slender and enveloped withinthe same skin, so that they appear like a single toe furnished with twoclaws. Notwithstanding this similarity of pattern, it is obvious that thehind feet of these several animals are used for as widely differentpurposes as it is possible to conceive. The case is rendered all the morestriking by the American opossums, which follow nearly the same habits oflife as some of their Australian relatives, having feet constructed on theordinary plan. Professor Flower, from whom these statements are taken,remarks in conclusion: "We may call this conformity to type, withoutgetting much nearer to an explanation of the phenomenon;" and he then adds"but is it not powerfully suggestive of true relationship, of inheritancefrom a common ancestor?"

Geoffroy St. Hilaire has strongly insisted on the high importance ofrelative position or connexion in homologous parts; they may differ toalmost any extent in form and size, and yet remain connected together inthe same invariable order. We never find, for instance, the bones of thearm and forearm, or of the thigh and leg, transposed. Hence the same namescan be given to the homologous bones in widely different animals. We seethe same great law in the construction of the mouths of insects: what canbe more different than the immensely long spiral proboscis of asphinx-moth, the curious folded one of a bee or bug, and the great jaws ofa beetle? Yet all these organs, serving for such widely differentpurposes, are formed by infinitely numerous modifications of an upper lip,mandibles, and two pairs of maxillae. The same law governs theconstruction of the mouths and limbs of crustaceans. So it is with theflowers of plants.

Nothing can be more hopeless than to attempt to explain this similarity ofpattern in members of the same class, by utility or by the doctrine offinal causes. The hopelessness of the attempt has been expressly admittedby Owen in his most interesting work on the "Nature of Limbs." On theordinary view of the independent creation of each being, we can only saythat so it is; that it has pleased the Creator to construct all the animalsand plants in each great class on a uniform plan; but this is not ascientific explanation.

The explanation is to a large extent simple, on the theory of the selectionof successive slight modifications, each being profitable in some way tothe modified form, but often affecting by correlation other parts of theorganisation. In changes of this nature, there will be little or notendency to alter the original pattern, or to transpose the parts. Thebones of a limb might be shortened and flattened to any extent, becoming atthe same time enveloped in thick membrane, so as to serve as a fin; or awebbed hand might have all its bones, or certain bones, lengthened to anyextent, with the membrane connecting them increased, so as to serve as awing; yet all these modifications would not tend to alter the framework ofthe bones or the relative connexion of the parts. If we suppose that anearly progenitor--the archetype, as it may be called--of all mammals, birdsand reptiles, had its limbs constructed on the existing general pattern,for whatever purpose they served, we can at once perceive the plainsignification of the homologous construction of the limbs throughout theclass. So with the mouths of insects, we have only to suppose that theircommon progenitor had an upper lip, mandibles, and two pairs of maxillae,these parts being perhaps very simple in form; and then natural selectionwill account for the infinite diversity in structure and function of themouths of insects. Nevertheless, it is conceivable that the generalpattern of an organ might become so much obscured as to be finally lost, bythe reduction and ultimately by the complete abortion of certain parts, bythe fusion of other parts, and by the doubling or multiplication of others,variations which we know to be within the limits of possibility. In thepaddles of the gigantic extinct sea-lizards, and in the mouths of certainsuctorial crustaceans, the general pattern seems thus to have becomepartially obscured.

There is another and equally curious branch of our subject; namely, serialhomologies, or the comparison of the different parts or organs in the sameindividual, and not of the same parts or organs in different members of thesame class. Most physiologists believe that the bones of the skull arehomologous--that is, correspond in number and in relative connexion--withthe elemental parts of a certain number of vertebrae. The anterior andposterior limbs in all the higher vertebrate classes are plainlyhomologous. So it is with the wonderfully complex jaws and legs ofcrustaceans. It is familiar to almost every one, that in a flower therelative position of the sepals, petals, stamens, and pistils, as well astheir intimate structure, are intelligible on the view that they consist ofmetamorphosed leaves, arranged in a spire. In monstrous plants, we oftenget direct evidence of the possibility of one organ being transformed intoanother; and we can actually see, during the early or embryonic stages ofdevelopment in flowers, as well as in crustaceans and many other animals,that organs, which when mature become extremely different are at firstexactly alike.

How inexplicable are the cases of serial homologies on the ordinary view ofcreation! Why should the brain be enclosed in a box composed of suchnumerous and such extraordinarily shaped pieces of bone apparentlyrepresenting vertebrae? As Owen has remarked, the benefit derived from theyielding of the separate pieces in the act of parturition by mammals, willby no means explain the same construction in the skulls of birds andreptiles. Why should similar bones have been created to form the wing andthe leg of a bat, used as they are for such totally different purposes,namely flying and walking? Why should one crustacean, which has anextremely complex mouth formed of many parts, consequently always havefewer legs; or conversely, those with many legs have simpler mouths? Whyshould the sepals, petals, stamens, and pistils, in each flower, thoughfitted for such distinct purposes, be all constructed on the same pattern?

On the theory of natural selection, we can, to a certain extent, answerthese questions. We need not here consider how the bodies of some animalsfirst became divided into a series of segments, or how they became dividedinto right and left sides, with corresponding organs, for such questionsare almost beyond investigation. It is, however, probable that some serialstructures are the result of cells multiplying by division, entailing themultiplication of the parts developed from such cells. It must suffice forour purpose to bear in mind that an indefinite repetition of the same partor organ is the common characteristic, as Owen has remarked, of all low orlittle specialised forms; therefore the unknown progenitor of theVertebrata probably possessed many vertebrae; the unknown progenitor of theArticulata, many segments; and the unknown progenitor of flowering plants,many leaves arranged in one or more spires. We have also formerly seenthat parts many times repeated are eminently liable to vary, not only innumber, but in form. Consequently such parts, being already present inconsiderable numbers, and being highly variable, would naturally afford thematerials for adaptation to the most different purposes; yet they wouldgenerally retain, through the force of inheritance, plain traces of theiroriginal or fundamental resemblance. They would retain this resemblanceall the more, as the variations, which afforded the basis for theirsubsequent modification through natural selection, would tend from thefirst to be similar; the parts being at an early stage of growth alike, andbeing subjected to nearly the same conditions. Such parts, whether more orless modified, unless their common origin became wholly obscured, would beserially homologous.

In the great class of molluscs, though the parts in distinct species can beshown to be homologous, only a few serial homologies; such as the valves ofChitons, can be indicated; that is, we are seldom enabled to say that onepart is homologous with another part in the same individual. And we canunderstand this fact; for in molluscs, even in the lowest members of theclass, we do not find nearly so much indefinite repetition of any one partas we find in the other great classes of the animal and vegetable kingdoms.

But morphology is a much more complex subject than it at first appears, ashas lately been well shown in a remarkable paper by Mr. E. Ray Lankester,who has drawn an important distinction between certain classes of caseswhich have all been equally ranked by naturalists as homologous. Heproposes to call the structures which resemble each other in distinctanimals, owing to their descent from a common progenitor with subsequentmodification, "homogenous"; and the resemblances which cannot thus beaccounted for, he proposes to call "homoplastic". For instance, hebelieves that the hearts of birds and mammals are as a whole homogenous--that is, have been derived from a common progenitor; but that the fourcavities of the heart in the two classes are homoplastic--that is, havebeen independently developed. Mr. Lankester also adduces the closeresemblance of the parts on the right and left sides of the body, and inthe successive segments of the same individual animal; and here we haveparts commonly called homologous which bear no relation to the descent ofdistinct species from a common progenitor. Homoplastic structures are thesame with those which I have classed, though in a very imperfect manner, asanalogous modifications or resemblances. Their formation may be attributedin part to distinct organisms, or to distinct parts of the same organism,having varied in an analogous manner; and in part to similar modifications,having been preserved for the same general purpose or function, of whichmany instances have been given.

Naturalists frequently speak of the skull as formed of metamorphosedvertebrae; the jaws of crabs as metamorphosed legs; the stamens and pistilsin flowers as metamorphosed leaves; but it would in most cases be morecorrect, as Professor Huxley has remarked, to speak of both skull andvertebrae, jaws and legs, etc., as having been metamorphosed, not one fromthe other, as they now exist, but from some common and simpler element. Most naturalists, however, use such language only in a metaphorical sense: they are far from meaning that during a long course of descent, primordialorgans of any kind--vertebrae in the one case and legs in the other--haveactually been converted into skulls or jaws. Yet so strong is theappearance of this having occurred that naturalists can hardly avoidemploying language having this plain signification. According to the viewshere maintained, such language may be used literally; and the wonderfulfact of the jaws, for instance, of a crab retaining numerous characters,which they probably would have retained through inheritance, if they hadreally been metamorphosed from true though extremely simple legs, is inpart explained.

DEVELOPMENT AND EMBRYOLOGY.

This is one of the most important subjects in the whole round of naturalhistory. The metamorphoses of insects, with which every one is familiar,are generally effected abruptly by a few stages; but the transformationsare in reality numerous and gradual, though concealed. A certainephemerous insect (Chloeon) during its development, moults, as shown by SirJ. Lubbock, above twenty times, and each time undergoes a certain amount ofchange; and in this case we see the act of metamorphosis performed in aprimary and gradual manner. Many insects, and especially certaincrustaceans, show us what wonderful changes of structure can be effectedduring development. Such changes, however, reach their acme in the so-called alternate generations of some of the lower animals. It is, forinstance, an astonishing fact that a delicate branching coralline, studdedwith polypi, and attached to a submarine rock, should produce, first bybudding and then by transverse division, a host of huge floating jelly-fishes; and that these should produce eggs, from which are hatched swimminganimalcules, which attach themselves to rocks and become developed intobranching corallines; and so on in an endless cycle. The belief in theessential identity of the process of alternate generation and of ordinarymetamorphosis has been greatly strengthened by Wagner's discovery of thelarva or maggot of a fly, namely the Cecidomyia, producing asexually otherlarvae, and these others, which finally are developed into mature males andfemales, propagating their kind in the ordinary manner by eggs.

It may be worth notice that when Wagner's remarkable discovery was firstannounced, I was asked how was it possible to account for the larvae ofthis fly having acquired the power of a sexual reproduction. As long asthe case remained unique no answer could be given. But already Grimm hasshown that another fly, a Chironomus, reproduces itself in nearly the samemanner, and he believes that this occurs frequently in the order. It isthe pupa, and not the larva, of the Chironomus which has this power; andGrimm further shows that this case, to a certain extent, "unites that ofthe Cecidomyia with the parthenogenesis of the Coccidae;" the termparthenogenesis implying that the mature females of the Coccidae arecapable of producing fertile eggs without the concourse of the male. Certain animals belonging to several classes are now known to have thepower of ordinary reproduction at an unusually early age; and we have onlyto accelerate parthenogenetic reproduction by gradual steps to an earlierand earlier age--Chironomus showing us an almost exactly intermediatestage, viz., that of the pupa--and we can perhaps account for themarvellous case of the Cecidomyia.

It has already been stated that various parts in the same individual, whichare exactly alike during an early embryonic period, become widely differentand serve for widely different purposes in the adult state. So again ithas been shown that generally the embryos of the most distinct speciesbelonging to the same class are closely similar, but become, when fullydeveloped, widely dissimilar. A better proof of this latter fact cannot begiven than the statement by Von Baer that "the embryos of mammalia, ofbirds, lizards and snakes, probably also of chelonia, are in the earlieststates exceedingly like one another, both as a whole and in the mode ofdevelopment of their parts; so much so, in fact, that we can oftendistinguish the embryos only by their size. In my possession are twolittle embryos in spirit, whose names I have omitted to attach, and atpresent I am quite unable to say to what class they belong. They may belizards or small birds, or very young mammalia, so complete is thesimilarity in the mode of formation of the head and trunk in these animals.The extremities, however, are still absent in these embryos. But even ifthey had existed in the earliest stage of their development we should learnnothing, for the feet of lizards and mammals, the wings and feet of birds,no less than the hands and feet of man, all arise from the same fundamentalform." The larvae of most crustaceans, at corresponding stages ofdevelopment, closely resemble each other, however different the adults maybecome; and so it is with very many other animals. A trace of the law ofembryonic resemblance occasionally lasts till a rather late age: thusbirds of the same genus, and of allied genera, often resemble each other intheir immature plumage; as we see in the spotted feathers in the young ofthe thrush group. In the cat tribe, most of the species when adult arestriped or spotted in lines; and stripes or spots can be plainlydistinguished in the whelp of the lion and the puma. We occasionally,though rarely, see something of the same kind in plants; thus the firstleaves of the ulex or furze, and the first leaves of the phyllodineousacacias, are pinnate or divided like the ordinary leaves of theleguminosae.

The points of structure, in which the embryos of widely different animalswithin the same class resemble each other, often have no direct relation totheir conditions of existence. We cannot, for instance, suppose that inthe embryos of the vertebrata the peculiar loop-like courses of thearteries near the branchial slits are related to similar conditions--in theyoung mammal which is nourished in the womb of its mother, in the egg ofthe bird which is hatched in a nest, and in the spawn of a frog underwater. We have no more reason to believe in such a relation than we haveto believe that the similar bones in the hand of a man, wing of a bat, andfin of a porpoise, are related to similar conditions of life. No onesupposes that the stripes on the whelp of a lion, or the spots on the youngblackbird, are of any use to these animals.

The case, however, is different when an animal, during any part of itsembryonic career, is active, and has to provide for itself. The period ofactivity may come on earlier or later in life; but whenever it comes on,the adaptation of the larva to its conditions of life is just as perfectand as beautiful as in the adult animal. In how important a manner thishas acted, has recently been well shown by Sir J. Lubbock in his remarks onthe close similarity of the larvae of some insects belonging to verydifferent orders, and on the dissimilarity of the larvae of other insectswithin the same order, according to their habits of life. Owing to suchadaptations the similarity of the larvae of allied animals is sometimesgreatly obscured; especially when there is a division of labour during thedifferent stages of development, as when the same larva has during onestage to search for food, and during another stage has to search for aplace of attachment. Cases can even be given of the larvae of alliedspecies, or groups of species, differing more from each other than do theadults. In most cases, however, the larvae, though active, still obey,more or less closely, the law of common embryonic resemblance. Cirripedesafford a good instance of this: even the illustrious Cuvier did notperceive that a barnacle was a crustacean: but a glance at the larva showsthis in an unmistakable manner. So again the two main divisions ofcirripedes, the pedunculated and sessile, though differing widely inexternal appearance, have larvae in all their stages barelydistinguishable.

The embryo in the course of development generally rises in organisation. Iuse this expression, though I am aware that it is hardly possible to defineclearly what is meant by organisation being higher or lower. But no oneprobably will dispute that the butterfly is higher than the caterpillar. In some cases, however, the mature animal must be considered as lower inthe scale than the larva, as with certain parasitic crustaceans. To referonce again to cirripedes: the larvae in the first stage have three pairsof locomotive organs, a simple single eye, and a probosciformed mouth, withwhich they feed largely, for they increase much in size. In the secondstage, answering to the chrysalis stage of butterflies, they have six pairsof beautifully constructed natatory legs, a pair of magnificent compoundeyes, and extremely complex antennae; but they have a closed and imperfectmouth, and cannot feed: their function at this stage is, to search out bytheir well-developed organs of sense, and to reach by their active powersof swimming, a proper place on which to become attached and to undergotheir final metamorphosis. When this is completed they are fixed for life: their legs are now converted into prehensile organs; they again obtain awell-constructed mouth; but they have no antennae, and their two eyes arenow reconverted into a minute, single, simple eye-spot. In this last andcomplete state, cirripedes may be considered as either more highly or morelowly organised than they were in the larval condition. But in some generathe larvae become developed into hermaphrodites having the ordinarystructure, or into what I have called complemental males; and in the latterthe development has assuredly been retrograde; for the male is a mere sack,which lives for a short time and is destitute of mouth, stomach, and everyother organ of importance, excepting those for reproduction.

We are so much accustomed to see a difference in structure between theembryo and the adult, that we are tempted to look at this difference as insome necessary manner contingent on growth. But there is no reason why,for instance, the wing of a bat, or the fin of a porpoise, should not havebeen sketched out with all their parts in proper proportion, as soon as anypart became visible. In some whole groups of animals and in certainmembers of other groups this is the case, and the embryo does not at anyperiod differ widely from the adult: thus Owen has remarked in regard tocuttle-fish, "there is no metamorphosis; the cephalopodic character ismanifested long before the parts of the embryo are completed." Land-shellsand fresh-water crustaceans are born having their proper forms, while themarine members of the same two great classes pass through considerable andoften great changes during their development. Spiders, again, barelyundergo any metamorphosis. The larvae of most insects pass through aworm-like stage, whether they are active and adapted to diversified habits,or are inactive from being placed in the midst of proper nutriment, or frombeing fed by their parents; but in some few cases, as in that of Aphis, ifwe look to the admirable drawings of the development of this insect, byProfessor Huxley, we see hardly any trace of the vermiform stage.

Sometimes it is only the earlier developmental stages which fail. Thus,Fritz Muller has made the remarkable discovery that certain shrimp-likecrustaceans (allied to Penoeus) first appear under the simple nauplius-form, and after passing through two or more zoea-stages, and then throughthe mysis-stage, finally acquire their mature structure: now in the wholegreat malacostracan order, to which these crustaceans belong, no othermember is as yet known to be first developed under the nauplius-form,though many appear as zoeas; nevertheless Muller assigns reasons for hisbelief, that if there had been no suppression of development, all thesecrustaceans would have appeared as nauplii.

How, then, can we explain these several facts in embryology--namely, thevery general, though not universal, difference in structure between theembryo and the adult; the various parts in the same individual embryo,which ultimately become very unlike, and serve for diverse purposes, beingat an early period of growth alike; the common, but not invariable,resemblance between the embryos or larvae of the most distinct species in the same class; the embryo often retaining, while within the egg or womb,structures which are of no service to it, either at that or at a laterperiod of life; on the other hand, larvae which have to provide for theirown wants, being perfectly adapted to the surrounding conditions; andlastly, the fact of certain larvae standing higher in the scale oforganisation than the mature animal into which they are developed? Ibelieve that all these facts can be explained as follows.

It is commonly assumed, perhaps from monstrosities affecting the embryo ata very early period, that slight variations or individual differencesnecessarily appear at an equally early period. We have little evidence onthis head, but what we have certainly points the other way; for it isnotorious that breeders of cattle, horses and various fancy animals, cannotpositively tell, until some time after birth, what will be the merits anddemerits of their young animals. We see this plainly in our own children;we cannot tell whether a child will be tall or short, or what its precisefeatures will be. The question is not, at what period of life anyvariation may have been caused, but at what period the effects aredisplayed. The cause may have acted, and I believe often has acted, on oneor both parents before the act of generation. It deserves notice that itis of no importance to a very young animal, as long as it is nourished andprotected by its parent, whether most of its characters are acquired alittle earlier or later in life. It would not signify, for instance, to abird which obtained its food by having a much-curved beak whether or notwhile young it possessed a beak of this shape, as long as it was fed by itsparents.

I have stated in the first chapter, that at whatever age any variationfirst appears in the parent, it tends to reappear at a corresponding age inthe offspring. Certain variations can only appear at corresponding ages;for instance, peculiarities in the caterpillar, cocoon, or imago states ofthe silk-moth; or, again, in the full-grown horns of cattle. Butvariations which, for all that we can see might have appeared eitherearlier or later in life, likewise tend to reappear at a corresponding agein the offspring and parent. I am far from meaning that this is invariablythe case, and I could give several exceptional cases of variations (takingthe word in the largest sense) which have supervened at an earlier age inthe child than in the parent.

These two principles, namely, that slight variations generally appear at anot very early period of life, and are inherited at a corresponding notearly period, explain, as I believe, all the above specified leading factsin embryology. But first let us look to a few analogous cases in ourdomestic varieties. Some authors who have written on Dogs maintain thatthe greyhound and bull-dog, though so different, are really closely alliedvarieties, descended from the same wild stock, hence I was curious to seehow far their puppies differed from each other. I was told by breedersthat they differed just as much as their parents, and this, judging by theeye, seemed almost to be the case; but on actually measuring the old dogsand their six-days-old puppies, I found that the puppies had not acquirednearly their full amount of proportional difference. So, again, I was toldthat the foals of cart and race-horses--breeds which have been almostwholly formed by selection under domestication--differed as much as thefull-grown animals; but having had careful measurements made of the damsand of three-days-old colts of race and heavy cart-horses, I find that thisis by no means the case.

As we have conclusive evidence that the breeds of the Pigeon are descendedfrom a single wild species, I compared the young pigeons within twelvehours after being hatched. I carefully measured the proportions (but willnot here give the details) of the beak, width of mouth, length of nostriland of eyelid, size of feet and length of leg, in the wild parent species,in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Now,some of these birds, when mature, differ in so extraordinary a manner inthe length and form of beak, and in other characters, that they wouldcertainly have been ranked as distinct genera if found in a state ofnature. But when the nestling birds of these several breeds were placed ina row, though most of them could just be distinguished, the proportionaldifferences in the above specified points were incomparably less than inthe full-grown birds. Some characteristic points of difference--forinstance, that of the width of mouth--could hardly be detected in theyoung. But there was one remarkable exception to this rule, for the youngof the short-faced tumbler differed from the young of the wild rock-pigeon,and of the other breeds, in almost exactly the same proportions as in theadult stage.

These facts are explained by the above two principles. Fanciers selecttheir dogs, horses, pigeons, etc., for breeding, when nearly grown up. They are indifferent whether the desired qualities are acquired earlier orlater in life, if the full-grown animal possesses them. And the cases justgiven, more especially that of the pigeons, show that the characteristicdifferences which have been accumulated by man's selection, and which givevalue to his breeds, do not generally appear at a very early period oflife, and are inherited at a corresponding not early period. But the caseof the short-faced tumbler, which when twelve hours old possessed itsproper characters, proves that this is not the universal rule; for here thecharacteristic differences must either have appeared at an earlier periodthan usual, or, if not so, the differences must have been inherited, not ata corresponding, but at an earlier age.

Now, let us apply these two principles to species in a state of nature. Let us take a group of birds, descended from some ancient form and modifiedthrough natural selection for different habits. Then, from the many slightsuccessive variations having supervened in the several species at a notearly age, and having been inherited at a corresponding age, the young willhave been but little modified, and they will still resemble each other muchmore closely than do the adults, just as we have seen with the breeds ofthe pigeon. We may extend this view to widely distinct structures and towhole classes. The fore-limbs, for instance, which once served as legs toa remote progenitor, may have become, through a long course ofmodification, adapted in one descendant to act as hands, in another aspaddles, in another as wings; but on the above two principles thefore-limbs will not have been much modified in the embryos of these severalforms; although in each form the fore-limb will differ greatly in the adultstate. Whatever influence long continued use or disuse may have had inmodifying the limbs or other parts of any species, this will chiefly orsolely have affected it when nearly mature, when it was compelled to useits full powers to gain its own living; and the effects thus produced willhave been transmitted to the offspring at a corresponding nearly matureage. Thus the young will not be modified, or will be modified only in aslight degree, through the effects of the increased use or disuse of parts.

With some animals the successive variations may have supervened at a veryearly period of life, or the steps may have been inherited at an earlierage than that at which they first occurred. In either of these cases theyoung or embryo will closely resemble the mature parent-form, as we haveseen with the short-faced tumbler. And this is the rule of development incertain whole groups, or in certain sub-groups alone, as with cuttle-fish,land-shells, fresh-water crustaceans, spiders, and some members of thegreat class of insects. With respect to the final cause of the young insuch groups not passing through any metamorphosis, we can see that thiswould follow from the following contingencies: namely, from the younghaving to provide at a very early age for their own wants, and from theirfollowing the same habits of life with their parents; for in this case itwould be indispensable for their existence that they should be modified inthe same manner as their parents. Again, with respect to the singular factthat many terrestrial and fresh-water animals do not undergo anymetamorphosis, while marine members of the same groups pass through varioustransformations, Fritz Muller has suggested that the process of slowlymodifying and adapting an animal to live on the land or in fresh water,instead of in the sea, would be greatly simplified by its not passingthrough any larval stage; for it is not probable that places well adaptedfor both the larval and mature stages, under such new and greatly changedhabits of life, would commonly be found unoccupied or ill-occupied by otherorganisms. In this case the gradual acquirement at an earlier and earlierage of the adult structure would be favoured by natural selection; and alltraces of former metamorphoses would finally be lost.

If, on the other hand, it profited the young of an animal to follow habitsof life slightly different from those of the parent-form, and consequentlyto be constructed on a slightly different plan, or if it profited a larvaalready different from its parent to change still further, then, on theprinciple of inheritance at corresponding ages, the young or the larvaemight be rendered by natural selection more and more different from theirparents to any conceivable extent. Differences in the larva might, also,become correlated with successive stages of its development; so that thelarva, in the first stage, might come to differ greatly from the larva inthe second stage, as is the case with many animals. The adult might alsobecome fitted for sites or habits, in which organs of locomotion or of thesenses, etc., would be useless; and in this case the metamorphosis would beretrograde.

>From the remarks just made we can see how by changes of structure in theyoung, in conformity with changed habits of life, together with inheritanceat corresponding ages, animals might come to pass through stages ofdevelopment, perfectly distinct from the primordial condition of theiradult progenitors. Most of our best authorities are now convinced that thevarious larval and pupal stages of insects have thus been acquired throughadaptation, and not through inheritance from some ancient form. Thecurious case of Sitaris--a beetle which passes through certain unusualstages of development--will illustrate how this might occur. The firstlarval form is described by M. Fabre, as an active, minute insect,furnished with six legs, two long antennae, and four eyes. These larvaeare hatched in the nests of bees; and when the male bees emerge from theirburrows, in the spring, which they do before the females, the larvae springon them, and afterwards crawl on to the females while paired with themales. As soon as the female bee deposits her eggs on the surface of thehoney stored in the cells, the larvae of the Sitaris leap on the eggs anddevour them. Afterwards they undergo a complete change; their eyesdisappear; their legs and antennae become rudimentary, and they feed onhoney; so that they now more closely resemble the ordinary larvae ofinsects; ultimately they undergo a further transformation, and finallyemerge as the perfect beetle. Now, if an insect, undergoingtransformations like those of the Sitaris, were to become the progenitor ofa whole new class of insects, the course of development of the new classwould be widely different from that of our existing insects; and the firstlarval stage certainly would not represent the former condition of anyadult and ancient form.

On the other hand it is highly probable that with many animals theembryonic or larval stages show us, more or less completely, the conditionof the progenitor of the whole group in its adult state. In the greatclass of the Crustacea, forms wonderfully distinct from each other, namely,suctorial parasites, cirripedes, entomostraca, and even the malacostraca,appear at first as larvae under the nauplius-form; and as these larvae liveand feed in the open sea, and are not adapted for any peculiar habits oflife, and from other reasons assigned by Fritz Muller, it is probable thatat some very remote period an independent adult animal, resembling theNauplius, existed, and subsequently produced, along several divergent linesof descent, the above-named great Crustacean groups. So again, it isprobable, from what we know of the embryos of mammals, birds, fishes andreptiles, that these animals are the modified descendants of some ancientprogenitor, which was furnished in its adult state with branchiae, a swim-bladder, four fin-like limbs, and a long tail, all fitted for an aquaticlife.

As all the organic beings, extinct and recent, which have ever lived, canbe arranged within a few great classes; and as all within each class have,according to our theory, been connected together by fine gradations, thebest, and, if our collections were nearly perfect, the only possiblearrangement, would be genealogical; descent being the hidden bond ofconnexion which naturalists have been seeking under the term of the NaturalSystem. On this view we can understand how it is that, in the eyes of mostnaturalists, the structure of the embryo is even more important forclassification than that of the adult. In two or more groups of animals,however much they may differ from each other in structure and habits intheir adult condition, if they pass through closely similar embryonicstages, we may feel assured that they are all descended from one parent-form, and are therefore closely related. Thus, community in embryonicstructure reveals community of descent; but dissimilarity in embryonicdevelopment does not prove discommunity of descent, for in one of twogroups the developmental stages may have been suppressed, or may have beenso greatly modified through adaptation to new habits of life as to be nolonger recognisable. Even in groups, in which the adults have beenmodified to an extreme degree, community of origin is often revealed by thestructure of the larvae; we have seen, for instance, that cirripedes,though externally so like shell-fish, are at once known by their larvae tobelong to the great class of crustaceans. As the embryo often shows usmore or less plainly the structure of the less modified and ancientprogenitor of the group, we can see why ancient and extinct forms so oftenresemble in their adult state the embryos of existing species of the sameclass. Agassiz believes this to be a universal law of nature; and we mayhope hereafter to see the law proved true. It can, however, be proved trueonly in those cases in which the ancient state of the progenitor of thegroup has not been wholly obliterated, either by successive variationshaving supervened at a very early period of growth, or by such variationshaving been inherited at an earlier age than that at which they firstappeared. It should also be borne in mind, that the law may be true, butyet, owing to the geological record not extending far enough back in time,may remain for a long period, or for ever, incapable of demonstration. Thelaw will not strictly hold good in those cases in which an ancient formbecame adapted in its larval state to some special line of life, andtransmitted the same larval state to a whole group of descendants; for suchlarval state will not resemble any still more ancient form in its adultstate.

Thus, as it seems to me, the leading facts in embryology, which are secondto none in importance, are explained on the principle of variations in themany descendants from some one ancient progenitor, having appeared at a notvery early period of life, and having been inherited at a correspondingperiod. Embryology rises greatly in interest, when we look at the embryoas a picture, more or less obscured, of the progenitor, either in its adultor larval state, of all the members of the same great class.

RUDIMENTARY, ATROPHIED, AND ABORTED ORGANS.

Organs or parts in this strange condition, bearing the plain stamp ofinutility, are extremely common, or even general, throughout nature. Itwould be impossible to name one of the higher animals in which some part orother is not in a rudimentary condition. In the mammalia, for instance,the males possess rudimentary mammae; in snakes one lobe of the lungs isrudimentary; in birds the "bastard-wing" may safely be considered as arudimentary digit, and in some species the whole wing is so far rudimentarythat it cannot be used for flight. What can be more curious than thepresence of teeth in foetal whales, which when grown up have not a tooth intheir heads; or the teeth, which never cut through the gums, in the upperjaws of unborn calves?

Rudimentary organs plainly declare their origin and meaning in variousways. There are beetles belonging to closely allied species, or even tothe same identical species, which have either full-sized and perfect wings,or mere rudiments of membrane, which not rarely lie under wing-coversfirmly soldered together; and in these cases it is impossible to doubt,that the rudiments represent wings. Rudimentary organs sometimes retaintheir potentiality: this occasionally occurs with the mammae of malemammals, which have been known to become well developed and to secretemilk. So again in the udders of the genus Bos, there are normally fourdeveloped and two rudimentary teats; but the latter in our domestic cowssometimes become well developed and yield milk. In regard to plants, thepetals are sometimes rudimentary, and sometimes well developed in theindividuals of the same species. In certain plants having separated sexesKolreuter found that by crossing a species, in which the male flowersincluded a rudiment of a pistil, with an hermaphrodite species, having ofcourse a well-developed pistil, the rudiment in the hybrid offspring wasmuch increased in size; and this clearly shows that the rudimentary andperfect pistils are essentially alike in nature. An animal may possessvarious parts in a perfect state, and yet they may in one sense berudimentary, for they are useless: thus the tadpole of the commonsalamander or water-newt, as Mr. G.H. Lewes remarks, "has gills, and passesits existence in the water; but the Salamandra atra, which lives high upamong the mountains, brings forth its young full-formed. This animal neverlives in the water. Yet if we open a gravid female, we find tadpolesinside her with exquisitely feathered gills; and when placed in water theyswim about like the tadpoles of the water-newt. Obviously this aquaticorganisation has no reference to the future life of the animal, nor has itany adaptation to its embryonic condition; it has solely reference toancestral adaptations, it repeats a phase in the development of itsprogenitors."

An organ, serving for two purposes, may become rudimentary or utterlyaborted for one, even the more important purpose, and remain perfectlyefficient for the other. Thus, in plants, the office of the pistil is toallow the pollen-tubes to reach the ovules within the ovarium. The pistilconsists of a stigma supported on the style; but in some Compositae, themale florets, which of course cannot be fecundated, have a rudimentarypistil, for it is not crowned with a stigma; but the style remains welldeveloped and is clothed in the usual manner with hairs, which serve tobrush the pollen out of the surrounding and conjoined anthers. Again, anorgan may become rudimentary for its proper purpose, and be used for adistinct one: in certain fishes the swim-bladder seems to be rudimentaryfor its proper function of giving buoyancy, but has become converted into anascent breathing organ or lung. Many similar instances could be given.

Useful organs, however little they may be developed, unless we have reasonto suppose that they were formerly more highly developed, ought not to beconsidered as rudimentary. They may be in a nascent condition, and inprogress towards further development. Rudimentary organs, on the otherhand, are either quite useless, such as teeth which never cut through thegums, or almost useless, such as the wings of an ostrich, which servemerely as sails. As organs in this condition would formerly, when stillless developed, have been of even less use than at present, they cannotformerly have been produced through variation and natural selection, whichacts solely by the preservation of useful modifications. They have beenpartially retained by the power of inheritance, and relate to a formerstate of things. It is, however, often difficult to distinguish betweenrudimentary and nascent organs; for we can judge only by analogy whether apart is capable of further development, in which case alone it deserves tobe called nascent. Organs in this condition will always be somewhat rare;for beings thus provided will commonly have been supplanted by theirsuccessors with the same organ in a more perfect state, and consequentlywill have become long ago extinct. The wing of the penguin is of highservice, acting as a fin; it may, therefore, represent the nascent state ofthe wing: not that I believe this to be the case; it is more probably areduced organ, modified for a new function: the wing of the Apteryx, onthe other hand, is quite useless, and is truly rudimentary. Owen considersthe simple filamentary limbs of the Lepidosiren as the "beginnings oforgans which attain full functional development in higher vertebrates;"but, according to the view lately advocated by Dr. Gunther, they areprobably remnants, consisting of the persistent axis of a fin, with thelateral rays or branches aborted. The mammary glands of theOrnithorhynchus may be considered, in comparison with the udders of a cow,as in a nascent condition. The ovigerous frena of certain cirripedes,which have ceased to give attachment to the ova and are feebly developed,are nascent branchiae.

Rudimentary organs in the individuals of the same species are very liableto vary in the degree of their development and in other respects. Inclosely allied species, also, the extent to which the same organ has beenreduced occasionally differs much. This latter fact is well exemplified inthe state of the wings of female moths belonging to the same family. Rudimentary organs may be utterly aborted; and this implies, that incertain animals or plants, parts are entirely absent which analogy wouldlead us to expect to find in them, and which are occasionally found inmonstrous individuals. Thus in most of the Scrophulariaceae the fifthstamen is utterly aborted; yet we may conclude that a fifth stamen onceexisted, for a rudiment of it is found in many species of the family, andthis rudiment occasionally becomes perfectly developed, as may sometimes beseen in the common snap-dragon. In tracing the homologies of any part indifferent members of the same class, nothing is more common, or, in orderfully to understand the relations of the parts, more useful than thediscovery of rudiments. This is well shown in the drawings given by Owenof the leg bones of the horse, ox, and rhinoceros.

It is an important fact that rudimentary organs, such as teeth in the upperjaws of whales and ruminants, can often be detected in the embryo, butafterwards wholly disappear. It is also, I believe, a universal rule, thata rudimentary part is of greater size in the embryo relatively to theadjoining parts, than in the adult; so that the organ at this early age isless rudimentary, or even cannot be said to be in any degree rudimentary. Hence rudimentary organs in the adult are often said to have retained theirembryonic condition.

I have now given the leading facts with respect to rudimentary organs. Inreflecting on them, every one must be struck with astonishment; for thesame reasoning power which tells us that most parts and organs areexquisitely adapted for certain purposes, tells us with equal plainnessthat these rudimentary or atrophied organs are imperfect and useless. Inworks on natural history, rudimentary organs are generally said to havebeen created "for the sake of symmetry," or in order "to complete thescheme of nature." But this is not an explanation, merely a restatement ofthe fact. Nor is it consistent with itself: thus the boa-constrictor hasrudiments of hind limbs and of a pelvis, and if it be said that these boneshave been retained "to complete the scheme of nature," why, as ProfessorWeismann asks, have they not been retained by other snakes, which do notpossess even a vestige of these same bones? What would be thought of anastronomer who maintained that the satellites revolve in elliptic coursesround their planets "for the sake of symmetry," because the planets thusrevolve round the sun? An eminent physiologist accounts for the presenceof rudimentary organs, by supposing that they serve to excrete matter inexcess, or matter injurious to the system; but can we suppose that theminute papilla, which often represents the pistil in male flowers, andwhich is formed of mere cellular tissue, can thus act? Can we suppose thatrudimentary teeth, which are subsequently absorbed, are beneficial to therapidly growing embryonic calf by removing matter so precious as phosphateof lime? When a man's fingers have been amputated, imperfect nails havebeen known to appear on the stumps, and I could as soon believe that thesevestiges of nails are developed in order to excrete horny matter, as thatthe rudimentary nails on the fin of the manatee have been developed forthis same purpose.

On the view of descent with modification, the origin of rudimentary organsis comparatively simple; and we can understand to a large extent the lawsgoverning their imperfect development. We have plenty of cases ofrudimentary organs in our domestic productions, as the stump of a tail intailless breeds, the vestige of an ear in earless breeds of sheep--thereappearance of minute dangling horns in hornless breeds of cattle, moreespecially, according to Youatt, in young animals--and the state of thewhole flower in the cauliflower. We often see rudiments of various partsin monsters; but I doubt whether any of these cases throw light on theorigin of rudimentary organs in a state of nature, further than by showingthat rudiments can be produced; for the balance of evidence clearlyindicates that species under nature do not undergo great and abruptchanges. But we learn from the study of our domestic productions that thedisuse of parts leads to their reduced size; and that the result isinherited.

It appears probable that disuse has been the main agent in rendering organsrudimentary. It would at first lead by slow steps to the more and morecomplete reduction of a part, until at last it became rudimentary--as inthe case of the eyes of animals inhabiting dark caverns, and of the wingsof birds inhabiting oceanic islands, which have seldom been forced bybeasts of prey to take flight, and have ultimately lost the power offlying. Again, an organ, useful under certain conditions, might becomeinjurious under others, as with the wings of beetles living on small andexposed islands; and in this case natural selection will have aided inreducing the organ, until it was rendered harmless and rudimentary.

Any change in structure and function, which can be effected by smallstages, is within the power of natural selection; so that an organrendered, through changed habits of life, useless or injurious for onepurpose, might be modified and used for another purpose. An organ might,also, be retained for one alone of its former functions. Organs,originally formed by the aid of natural selection, when rendered uselessmay well be variable, for their variations can no longer be checked bynatural selection. All this agrees well with what we see under nature. Moreover, at whatever period of life either disuse or selection reduces anorgan, and this will generally be when the being has come to maturity andto exert its full powers of action, the principle of inheritance atcorresponding ages will tend to reproduce the organ in its reduced state atthe same mature age, but will seldom affect it in the embryo. Thus we canunderstand the greater size of rudimentary organs in the embryo relativelyto the adjoining parts, and their lesser relative size in the adult. If,for instance, the digit of an adult animal was used less and less duringmany generations, owing to some change of habits, or if an organ or glandwas less and less functionally exercised, we may infer that it would becomereduced in size in the adult descendants of this animal, but would retainnearly its original standard of development in the embryo.

There remains, however, this difficulty. After an organ has ceased beingused, and has become in consequence much reduced, how can it be stillfurther reduced in size until the merest vestige is left; and how can it befinally quite obliterated? It is scarcely possible that disuse can go onproducing any further effect after the organ has once been renderedfunctionless. Some additional explanation is here requisite which I cannotgive. If, for instance, it could be proved that every part of theorganisation tends to vary in a greater degree towards diminution thantoward augmentation of size, then we should be able to understand how anorgan which has become useless would be rendered, independently of theeffects of disuse, rudimentary and would at last be wholly suppressed; forthe variations towards diminished size would no longer be checked bynatural selection. The principle of the economy of growth, explained in aformer chapter, by which the materials forming any part, if not useful tothe possessor, are saved as far as is possible, will perhaps come into playin rendering a useless part rudimentary. But this principle will almostnecessarily be confined to the earlier stages of the process of reduction;for we cannot suppose that a minute papilla, for instance, representing ina male flower the pistil of the female flower, and formed merely ofcellular tissue, could be further reduced or absorbed for the sake ofeconomising nutriment.

Finally, as rudimentary organs, by whatever steps they may have beendegraded into their present useless condition, are the record of a formerstate of things, and have been retained solely through the power ofinheritance--we can understand, on the genealogical view of classification,how it is that systematists, in placing organisms in their proper places inthe natural system, have often found rudimentary parts as useful as, oreven sometimes more useful than, parts of high physiological importance. Rudimentary organs may be compared with the letters in a word, stillretained in the spelling, but become useless in the pronunciation, butwhich serve as a clue for its derivation. On the view of descent withmodification, we may conclude that the existence of organs in arudimentary, imperfect, and useless condition, or quite aborted, far frompresenting a strange difficulty, as they assuredly do on the old doctrineof creation, might even have been anticipated in accordance with the viewshere explained.

SUMMARY.

In this chapter I have attempted to show that the arrangement of allorganic beings throughout all time in groups under groups--that the natureof the relationships by which all living and extinct organisms are unitedby complex, radiating, and circuitous lines of affinities into a few grandclasses--the rules followed and the difficulties encountered by naturalistsin their classifications--the value set upon characters, if constant andprevalent, whether of high or of the most trifling importance, or, as withrudimentary organs of no importance--the wide opposition in value betweenanalogical or adaptive characters, and characters of true affinity; andother such rules--all naturally follow if we admit the common parentage ofallied forms, together with their modification through variation andnatural selection, with the contingencies of extinction and divergence ofcharacter. In considering this view of classification, it should be bornein mind that the element of descent has been universally used in rankingtogether the sexes, ages, dimorphic forms, and acknowledged varieties ofthe same species, however much they may differ from each other instructure. If we extend the use of this element of descent--the onecertainly known cause of similarity in organic beings--we shall understandwhat is meant by the Natural System: it is genealogical in its attemptedarrangement, with the grades of acquired difference marked by the terms,varieties, species, genera, families, orders, and classes.

On this same view of descent with modification, most of the great facts inMorphology become intelligible--whether we look to the same patterndisplayed by the different species of the same class in their homologousorgans, to whatever purpose applied, or to the serial and lateralhomologies in each individual animal and plant.

On the principle of successive slight variations, not necessarily orgenerally supervening at a very early period of life, and being inheritedat a corresponding period, we can understand the leading facts inembryology; namely, the close resemblance in the individual embryo of theparts which are homologous, and which when matured become widely differentin structure and function; and the resemblance of the homologous parts ororgans in allied though distinct species, though fitted in the adult statefor habits as different as is possible. Larvae are active embryos, whichhave become specially modified in a greater or less degree in relation totheir habits of life, with their modifications inherited at a correspondingearly age. On these same principles, and bearing in mind that when organsare reduced in size, either from disuse or through natural selection, itwill generally be at that period of life when the being has to provide forits own wants, and bearing in mind how strong is the force ofinheritance--the occurrence of rudimentary organs might even have beenanticipated. The importance of embryological characters and of rudimentaryorgans in classification is intelligible, on the view that a naturalarrangement must be genealogical.

Finally, the several classes of facts which have been considered in thischapter, seem to me to proclaim so plainly, that the innumerable species,genera and families, with which this world is peopled, are all descended,each within its own class or group, from common parents, and have all beenmodified in the course of descent, that I should without hesitation adoptthis view, even if it were unsupported by other facts or arguments.