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Effects of changed conditions -- Use and disuse, combined with naturalselection; organs of flight and of vision -- Acclimatisation -- Correlatedvariation -- Compensation and economy of growth -- False correlations --Multiple, rudimentary, and lowly organised structures variable -- Partsdeveloped in an unusual manner are highly variable: specific charactersmore variable than generic: secondary sexual characters variable --Species of the same genus vary in an analogous manner -- Reversions tolong-lost characters -- Summary.

I have hitherto sometimes spoken as if the variations--so common andmultiform with organic beings under domestication, and in a lesser degreewith those under nature--were due to chance. This, of course is a whollyincorrect expression, but it serves to acknowledge plainly our ignorance ofthe cause of each particular variation. Some authors believe it to be asmuch the function of the reproductive system to produce individualdifferences, or slight deviations of structure, as to make the child likeits parents. But the fact of variations and monstrosities occurring muchmore frequently under domestication than under nature, and the greatervariability of species having wide ranges than of those with restrictedranges, lead to the conclusion that variability is generally related to theconditions of life to which each species has been exposed during severalsuccessive generations. In the first chapter I attempted to show thatchanged conditions act in two ways, directly on the whole organisation oron certain parts alone, and indirectly through the reproductive system. Inall cases there are two factors, the nature of the organism, which is muchthe most important of the two, and the nature of the conditions. Thedirect action of changed conditions leads to definite or indefiniteresults. In the latter case the organisation seems to become plastic, andwe have much fluctuating variability. In the former case the nature of theorganism is such that it yields readily, when subjected to certainconditions, and all, or nearly all, the individuals become modified in thesame way.

It is very difficult to decide how far changed conditions, such as ofclimate, food, etc., have acted in a definite manner. There is reason tobelieve that in the course of time the effects have been greater than canbe proved by clear evidence. But we may safely conclude that theinnumerable complex co-adaptations of structure, which we see throughoutnature between various organic beings, cannot be attributed simply to suchaction. In the following cases the conditions seem to have produced someslight definite effect: E. Forbes asserts that shells at their southernlimit, and when living in shallow water, are more brightly coloured thanthose of the same species from further north or from a greater depth; butthis certainly does not always hold good. Mr. Gould believes that birds ofthe same species are more brightly coloured under a clear atmosphere, thanwhen living near the coast or on islands; and Wollaston is convinced thatresidence near the sea affects the colours of insects. Moquin-Tandon givesa list of plants which, when growing near the sea-shore, have their leavesin some degree fleshy, though not elsewhere fleshy. These slightly varyingorganisms are interesting in as far as they present characters analogous tothose possessed by the species which are confined to similar conditions.

When a variation is of the slightest use to any being, we cannot tell howmuch to attribute to the accumulative action of natural selection, and howmuch to the definite action of the conditions of life. Thus, it is wellknown to furriers that animals of the same species have thicker and betterfur the further north they live; but who can tell how much of thisdifference may be due to the warmest-clad individuals having been favouredand preserved during many generations, and how much to the action of thesevere climate? For it would appear that climate has some direct action onthe hair of our domestic quadrupeds.

Instances could be given of similar varieties being produced from the samespecies under external conditions of life as different as can well beconceived; and, on the other hand, of dissimilar varieties being producedunder apparently the same external conditions. Again, innumerableinstances are known to every naturalist, of species keeping true, or notvarying at all, although living under the most opposite climates. Suchconsiderations as these incline me to lay less weight on the direct actionof the surrounding conditions, than on a tendency to vary, due to causes ofwhich we are quite ignorant.

In one sense the conditions of life may be said, not only to causevariability, either directly or indirectly, but likewise to include naturalselection, for the conditions determine whether this or that variety shallsurvive. But when man is the selecting agent, we clearly see that the twoelements of change are distinct; variability is in some manner excited, butit is the will of man which accumulates the variations in certaindirection; and it is this latter agency which answers to the survival ofthe fittest under nature.

EFFECTS OF THE INCREASED USE AND DISUSE OF PARTS, AS CONTROLLED BY NATURALSELECTION.

>From the facts alluded to in the first chapter, I think there can be nodoubt that use in our domestic animals has strengthened and enlargedcertain parts, and disuse diminished them; and that such modifications areinherited. Under free nature we have no standard of comparison by which tojudge of the effects of long-continued use or disuse, for we know not theparent-forms; but many animals possess structures which can be bestexplained by the effects of disuse. As Professor Owen has remarked, thereis no greater anomaly in nature than a bird that cannot fly; yet there areseveral in this state. The logger-headed duck of South America can onlyflap along the surface of the water, and has its wings in nearly the samecondition as the domestic Aylesbury duck: it is a remarkable fact that theyoung birds, according to Mr. Cunningham, can fly, while the adults havelost this power. As the larger ground-feeding birds seldom take flightexcept to escape danger, it is probable that the nearly wingless conditionof several birds, now inhabiting or which lately inhabited several oceanicislands, tenanted by no beasts of prey, has been caused by disuse. Theostrich indeed inhabits continents, and is exposed to danger from which itcannot escape by flight, but it can defend itself, by kicking its enemies,as efficiently as many quadrupeds. We may believe that the progenitor ofthe ostrich genus had habits like those of the bustard, and that, as thesize and weight of its body were increased during successive generations,its legs were used more and its wings less, until they became incapable offlight.

Kirby has remarked (and I have observed the same fact) that the anteriortarsi, or feet, of many male dung-feeding beetles are often broken off; heexamined seventeen specimens in his own collection, and not one had even arelic left. In the Onites apelles the tarsi are so habitually lost thatthe insect has been described as not having them. In some other generathey are present, but in a rudimentary condition. In the Ateuchus orsacred beetle of the Egyptians, they are totally deficient. The evidencethat accidental mutilations can be inherited is at present not decisive;but the remarkable cases observed by Brown-Sequard in guinea-pigs, of theinherited effects of operations, should make us cautious in denying thistendency. Hence, it will perhaps be safest to look at the entire absenceof the anterior tarsi in Ateuchus, and their rudimentary condition in someother genera, not as cases of inherited mutilations, but as due to theeffects of long-continued disuse; for as many dung-feeding beetles aregenerally found with their tarsi lost, this must happen early in life;therefore the tarsi cannot be of much importance or be much used by theseinsects.

In some cases we might easily put down to disuse modifications of structurewhich are wholly, or mainly due to natural selection. Mr. Wollaston hasdiscovered the remarkable fact that 200 beetles, out of the 550 species(but more are now known) inhabiting Madeira, are so far deficient in wingsthat they cannot fly; and that, of the twenty-nine endemic genera, no lessthan twenty-three have all their species in this condition! Several facts,namely, that beetles in many parts of the world are very frequently blownto sea and perish; that the beetles in Madeira, as observed by Mr.Wollaston, lie much concealed, until the wind lulls and the sun shines;that the proportion of wingless beetles is larger on the exposed Desertasthan in Madeira itself; and especially the extraordinary fact, so stronglyinsisted on by Mr. Wollaston, that certain large groups of beetles,elsewhere excessively numerous, which absolutely require the use of theirwings, are here almost entirely absent. These several considerations makeme believe that the wingless condition of so many Madeira beetles is mainlydue to the action of natural selection, combined probably with disuse. Forduring many successive generations each individual beetle which flew least,either from its wings having been ever so little less perfectly developedor from indolent habit, will have had the best chance of surviving from notbeing blown out to sea; and, on the other hand, those beetles which mostreadily took to flight would oftenest have been blown to sea, and thusdestroyed.

The insects in Madeira which are not ground-feeders, and which, as certainflower-feeding coleoptera and lepidoptera, must habitually use their wingsto gain their subsistence, have, as Mr. Wollaston suspects, their wings notat all reduced, but even enlarged. This is quite compatible with theaction of natural selection. For when a new insect first arrived on theisland, the tendency of natural selection to enlarge or to reduce thewings, would depend on whether a greater number of individuals were savedby successfully battling with the winds, or by giving up the attempt andrarely or never flying. As with mariners shipwrecked near a coast, itwould have been better for the good swimmers if they had been able to swimstill further, whereas it would have been better for the bad swimmers ifthey had not been able to swim at all and had stuck to the wreck.

The eyes of moles and of some burrowing rodents are rudimentary in size,and in some cases are quite covered by skin and fur. This state of theeyes is probably due to gradual reduction from disuse, but aided perhaps bynatural selection. In South America, a burrowing rodent, the tuco-tuco, orCtenomys, is even more subterranean in its habits than the mole; and I wasassured by a Spaniard, who had often caught them, that they were frequentlyblind. One which I kept alive was certainly in this condition, the cause,as appeared on dissection, having been inflammation of the nictitatingmembrane. As frequent inflammation of the eyes must be injurious to anyanimal, and as eyes are certainly not necessary to animals havingsubterranean habits, a reduction in their size, with the adhesion of theeyelids and growth of fur over them, might in such case be an advantage;and if so, natural selection would aid the effects of disuse.

It is well known that several animals, belonging to the most differentclasses, which inhabit the caves of Carniola and Kentucky, are blind. Insome of the crabs the foot-stalk for the eye remains, though the eye isgone; the stand for the telescope is there, though the telescope with itsglasses has been lost. As it is difficult to imagine that eyes, thoughuseless, could be in any way injurious to animals living in darkness, theirloss may be attributed to disuse. In one of the blind animals, namely, thecave-rat (Neotoma), two of which were captured by Professor Silliman atabove half a mile distance from the mouth of the cave, and therefore not inthe profoundest depths, the eyes were lustrous and of large size; and theseanimals, as I am informed by Professor Silliman, after having been exposedfor about a month to a graduated light, acquired a dim perception ofobjects.

It is difficult to imagine conditions of life more similar than deeplimestone caverns under a nearly similar climate; so that, in accordancewith the old view of the blind animals having been separately created forthe American and European caverns, very close similarity in theirorganisation and affinities might have been expected. This is certainlynot the case if we look at the two whole faunas; with respect to theinsects alone, Schiodte has remarked: "We are accordingly prevented fromconsidering the entire phenomenon in any other light than something purelylocal, and the similarity which is exhibited in a few forms between theMammoth Cave (in Kentucky) and the caves in Carniola, otherwise than as avery plain expression of that analogy which subsists generally between thefauna of Europe and of North America." On my view we must suppose thatAmerican animals, having in most cases ordinary powers of vision, slowlymigrated by successive generations from the outer world into the deeper anddeeper recesses of the Kentucky caves, as did European animals into thecaves of Europe. We have some evidence of this gradation of habit; for, asSchiodte remarks: "We accordingly look upon the subterranean faunas assmall ramifications which have penetrated into the earth from thegeographically limited faunas of the adjacent tracts, and which, as theyextended themselves into darkness, have been accommodated to surroundingcircumstances. Animals not far remote from ordinary forms, prepare thetransition from light to darkness. Next follow those that are constructedfor twilight; and, last of all, those destined for total darkness, andwhose formation is quite peculiar." These remarks of Schiodte's it shouldbe understood, apply not to the same, but to distinct species. By the timethat an animal had reached, after numberless generations, the deepestrecesses, disuse will on this view have more or less perfectly obliteratedits eyes, and natural selection will often have effected other changes,such as an increase in the length of the antennae or palpi, as acompensation for blindness. Notwithstanding such modifications, we mightexpect still to see in the cave-animals of America, affinities to the otherinhabitants of that continent, and in those of Europe to the inhabitants ofthe European continent. And this is the case with some of the Americancave-animals, as I hear from Professor Dana; and some of the Europeancave-insects are very closely allied to those of the surrounding country. It would be difficult to give any rational explanation of the affinities ofthe blind cave-animals to the other inhabitants of the two continents onthe ordinary view of their independent creation. That several of theinhabitants of the caves of the Old and New Worlds should be closelyrelated, we might expect from the well-known relationship of most of theirother productions. As a blind species of Bathyscia is found in abundanceon shady rocks far from caves, the loss of vision in the cave species ofthis one genus has probably had no relation to its dark habitation; for itis natural that an insect already deprived of vision should readily becomeadapted to dark caverns. Another blind genus (Anophthalmus) offers thisremarkable peculiarity, that the species, as Mr. Murray observes, have notas yet been found anywhere except in caves; yet those which inhabit theseveral caves of Europe and America are distinct; but it is possible thatthe progenitors of these several species, while they were furnished witheyes, may formerly have ranged over both continents, and then have becomeextinct, excepting in their present secluded abodes. Far from feelingsurprise that some of the cave-animals should be very anomalous, as Agassizhas remarked in regard to the blind fish, the Amblyopsis, and as is thecase with the blind Proteus, with reference to the reptiles of Europe, I amonly surprised that more wrecks of ancient life have not been preserved,owing to the less severe competition to which the scanty inhabitants ofthese dark abodes will have been exposed.

ACCLIMATISATION.

Habit is hereditary with plants, as in the period of flowering, in the timeof sleep, in the amount of rain requisite for seeds to germinate, etc., andthis leads me to say a few words on acclimatisation. As it is extremelycommon for distinct species belonging to the same genus to inhabit hot andcold countries, if it be true that all the species of the same genus aredescended from a single parent-form, acclimatisation must be readilyeffected during a long course of descent. It is notorious that eachspecies is adapted to the climate of its own home: species from an arcticor even from a temperate region cannot endure a tropical climate, orconversely. So again, many succulent plants cannot endure a damp climate. But the degree of adaptation of species to the climates under which theylive is often overrated. We may infer this from our frequent inability topredict whether or not an imported plant will endure our climate, and fromthe number of plants and animals brought from different countries which arehere perfectly healthy. We have reason to believe that species in a stateof nature are closely limited in their ranges by the competition of otherorganic beings quite as much as, or more than, by adaptation to particularclimates. But whether or not this adaptation is in most cases very close,we have evidence with some few plants, of their becoming, to a certainextent, naturally habituated to different temperatures; that is, theybecome acclimatised: thus the pines and rhododendrons, raised from seedcollected by Dr. Hooker from the same species growing at different heightson the Himalayas, were found to possess in this country differentconstitutional powers of resisting cold. Mr. Thwaites informs me that hehas observed similar facts in Ceylon; analogous observations have been madeby Mr. H.C. Watson on European species of plants brought from the Azores toEngland; and I could give other cases. In regard to animals, severalauthentic instances could be adduced of species having largely extended,within historical times, their range from warmer to colder latitudes, andconversely; but we do not positively know that these animals were strictlyadapted to their native climate, though in all ordinary cases we assumesuch to be the case; nor do we know that they have subsequently becomespecially acclimatised to their new homes, so as to be better fitted forthem than they were at first.

As we may infer that our domestic animals were originally chosen byuncivilised man because they were useful, and because they bred readilyunder confinement, and not because they were subsequently found capable offar-extended transportation, the common and extraordinary capacity in ourdomestic animals of not only withstanding the most different climates, butof being perfectly fertile (a far severer test) under them, may be used asan argument that a large proportion of other animals now in a state ofnature could easily be brought to bear widely different climates. We mustnot, however, push the foregoing argument too far, on account of theprobable origin of some of our domestic animals from several wild stocks: the blood, for instance, of a tropical and arctic wolf may perhaps bemingled in our domestic breeds. The rat and mouse cannot be considered asdomestic animals, but they have been transported by man to many parts ofthe world, and now have a far wider range than any other rodent; for theylive under the cold climate of Faroe in the north and of the Falklands inthe south, and on many an island in the torrid zones. Hence adaptation toany special climate may be looked at as a quality readily grafted on aninnate wide flexibility of constitution, common to most animals. On thisview, the capacity of enduring the most different climates by man himselfand by his domestic animals, and the fact of the extinct elephant andrhinoceros having formerly endured a glacial climate, whereas the livingspecies are now all tropical or sub-tropical in their habits, ought not tobe looked at as anomalies, but as examples of a very common flexibility ofconstitution, brought, under peculiar circumstances, into action.

How much of the acclimatisation of species to any peculiar climate is dueto mere habit, and how much to the natural selection of varieties havingdifferent innate constitutions, and how much to both means combined, is anobscure question. That habit or custom has some influence, I must believe,both from analogy and from the incessant advice given in agriculturalworks, even in the ancient Encyclopaedias of China, to be very cautious intransporting animals from one district to another. And as it is not likelythat man should have succeeded in selecting so many breeds and sub-breedswith constitutions specially fitted for their own districts, the resultmust, I think, be due to habit. On the other hand, natural selection wouldinevitably tend to preserve those individuals which were born withconstitutions best adapted to any country which they inhabited. Intreatises on many kinds of cultivated plants, certain varieties are said towithstand certain climates better than others; this is strikingly shown inworks on fruit-trees published in the United States, in which certainvarieties are habitually recommended for the northern and others for thesouthern states; and as most of these varieties are of recent origin, theycannot owe their constitutional differences to habit. The case of theJerusalem artichoke, which is never propagated in England by seed, and ofwhich, consequently, new varieties have not been produced, has even beenadvanced, as proving that acclimatisation cannot be effected, for it is nowas tender as ever it was! The case, also, of the kidney-bean has beenoften cited for a similar purpose, and with much greater weight; but untilsome one will sow, during a score of generations, his kidney-beans so earlythat a very large proportion are destroyed by frost, and then collect seedfrom the few survivors, with care to prevent accidental crosses, and thenagain get seed from these seedlings, with the same precautions, theexperiment cannot be said to have been even tried. Nor let it be supposedthat differences in the constitution of seedling kidney-beans never appear,for an account has been published how much more hardy some seedlings arethan others; and of this fact I have myself observed striking instances.

On the whole, we may conclude that habit, or use and disuse, have, in somecases, played a considerable part in the modification of the constitutionand structure; but that the effects have often been largely combined with,and sometimes overmastered by, the natural selection of innate variations.

CORRELATED VARIATION.

I mean by this expression that the whole organisation is so tied together,during its growth and development, that when slight variations in any onepart occur and are accumulated through natural selection, other partsbecome modified. This is a very important subject, most imperfectlyunderstood, and no doubt wholly different classes of facts may be hereeasily confounded together. We shall presently see that simple inheritanceoften gives the false appearance of correlation. One of the most obviousreal cases is, that variations of structure arising in the young or larvaenaturally tend to affect the structure of the mature animal. The severalparts which are homologous, and which, at an early embryonic period, areidentical in structure, and which are necessarily exposed to similarconditions, seem eminently liable to vary in a like manner: we see this inthe right and left sides of the body varying in the same manner; in thefront and hind legs, and even in the jaws and limbs, varying together, forthe lower jaw is believed by some anatomists to be homologous with thelimbs. These tendencies, I do not doubt, may be mastered more or lesscompletely by natural selection: thus a family of stags once existed withan antler only on one side; and if this had been of any great use to thebreed, it might probably have been rendered permanent by natural selection.

Homologous parts, as has been remarked by some authors, tend to cohere;this is often seen in monstrous plants: and nothing is more common thanthe union of homologous parts in normal structures, as in the union of thepetals into a tube. Hard parts seem to affect the form of adjoining softparts; it is believed by some authors that with birds the diversity in theshape of the pelvis causes the remarkable diversity in the shape of thekidneys. Others believe that the shape of the pelvis in the human motherinfluences by pressure the shape of the head of the child. In snakes,according to Schlegel, the shape of the body and the manner of swallowingdetermine the position and form of several of the most important viscera.

The nature of the bond is frequently quite obscure. M. Is. Geoffroy St.Hilaire has forcibly remarked that certain malconformations frequently, andthat others rarely, coexist without our being able to assign any reason. What can be more singular than the relation in cats between completewhiteness and blue eyes with deafness, or between the tortoise-shell colourand the female sex; or in pigeons, between their feathered feet and skinbetwixt the outer toes, or between the presence of more or less down on theyoung pigeon when first hatched, with the future colour of its plumage; or,again, the relation between the hair and the teeth in the naked Turkishdog, though here no doubt homology comes into play? With respect to thislatter case of correlation, I think it can hardly be accidental that thetwo orders of mammals which are most abnormal in their dermal covering,viz., Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters, etc.),are likewise on the whole the most abnormal in their teeth, but there areso many exceptions to this rule, as Mr. Mivart has remarked, that it haslittle value.

I know of no case better adapted to show the importance of the laws ofcorrelation and variation, independently of utility, and therefore ofnatural selection, than that of the difference between the outer and innerflowers in some Compositous and Umbelliferous plants. Everyone is familiarwith the difference between the ray and central florets of, for instance,the daisy, and this difference is often accompanied with the partial orcomplete abortion of the reproductive organs. But in some of these plantsthe seeds also differ in shape and sculpture. These differences havesometimes been attributed to the pressure of the involucra on the florets,or to their mutual pressure, and the shape of the seeds in the ray-floretsof some Compositae countenances this idea; but with the Umbelliferae it isby no means, as Dr. Hooker informs me, the species with the densest headswhich most frequently differ in their inner and outer flowers. It mighthave been thought that the development of the ray-petals, by drawingnourishment from the reproductive organs causes their abortion; but thiscan hardly be the sole case, for in some Compositae the seeds of the outerand inner florets differ, without any difference in the corolla. Possiblythese several differences may be connected with the different flow ofnutriment towards the central and external flowers. We know, at least,that with irregular flowers those nearest to the axis are most subject topeloria, that is to become abnormally symmetrical. I may add, as aninstance of this fact, and as a striking case of correlation, that in manypelargoniums the two upper petals in the central flower of the truss oftenlose their patches of darker colour; and when this occurs, the adherentnectary is quite aborted, the central flower thus becoming peloric orregular. When the colour is absent from only one of the two upper petals,the nectary is not quite aborted but is much shortened.

With respect to the development of the corolla, Sprengel's idea that theray-florets serve to attract insects, whose agency is highly advantageous,or necessary for the fertilisation of these plants, is highly probable; andif so, natural selection may have come into play. But with respect to theseeds, it seems impossible that their differences in shape, which are notalways correlated with any difference in the corolla, can be in any waybeneficial; yet in the Umbelliferae these differences are of such apparentimportance--the seeds being sometimes orthospermous in the exterior flowersand coelospermous in the central flowers--that the elder De Candollefounded his main divisions in the order on such characters. Hencemodifications of structure, viewed by systematists as of high value, may bewholly due to the laws of variation and correlation, without being, as faras we can judge, of the slightest service to the species.

We may often falsely attribute to correlated variation structures which arecommon to whole groups of species, and which in truth are simply due toinheritance; for an ancient progenitor may have acquired through naturalselection some one modification in structure, and, after thousands ofgenerations, some other and independent modification; and these twomodifications, having been transmitted to a whole group of descendants withdiverse habits, would naturally be thought to be in some necessary mannercorrelated. Some other correlations are apparently due to the manner inwhich natural selection can alone act. For instance, Alph. De Candolle hasremarked that winged seeds are never found in fruits which do not open; Ishould explain this rule by the impossibility of seeds gradually becomingwinged through natural selection, unless the capsules were open; for inthis case alone could the seeds, which were a little better adapted to bewafted by the wind, gain an advantage over others less well fitted for widedispersal.

COMPENSATION AND ECONOMY OF GROWTH.

The elder Geoffroy and Goethe propounded, at about the same time, their lawof compensation or balancement of growth; or, as Goethe expressed it, "inorder to spend on one side, nature is forced to economise on the otherside." I think this holds true to a certain extent with our domesticproductions: if nourishment flows to one part or organ in excess, itrarely flows, at least in excess, to another part; thus it is difficult toget a cow to give much milk and to fatten readily. The same varieties ofthe cabbage do not yield abundant and nutritious foliage and a copioussupply of oil-bearing seeds. When the seeds in our fruits becomeatrophied, the fruit itself gains largely in size and quality. In ourpoultry, a large tuft of feathers on the head is generally accompanied by adiminished comb, and a large beard by diminished wattles. With species ina state of nature it can hardly be maintained that the law is of universalapplication; but many good observers, more especially botanists, believe inits truth. I will not, however, here give any instances, for I see hardlyany way of distinguishing between the effects, on the one hand, of a partbeing largely developed through natural selection and another and adjoiningpart being reduced by the same process or by disuse, and, on the otherhand, the actual withdrawal of nutriment from one part owing to the excessof growth in another and adjoining part.

I suspect, also, that some of the cases of compensation which have beenadvanced, and likewise some other facts, may be merged under a more generalprinciple, namely, that natural selection is continually trying toeconomise in every part of the organisation. If under changed conditionsof life a structure, before useful, becomes less useful, its diminutionwill be favoured, for it will profit the individual not to have itsnutriment wasted in building up a useless structure. I can thus onlyunderstand a fact with which I was much struck when examining cirripedes,and of which many other instances could be given: namely, that when acirripede is parasitic within another cirripede and is thus protected, itloses more or less completely its own shell or carapace. This is the casewith the male Ibla, and in a truly extraordinary manner with theProteolepas: for the carapace in all other cirripedes consists of thethree highly important anterior segments of the head enormously developed,and furnished with great nerves and muscles; but in the parasitic andprotected Proteolepas, the whole anterior part of the head is reduced tothe merest rudiment attached to the bases of the prehensile antennae. Nowthe saving of a large and complex structure, when rendered superfluous,would be a decided advantage to each successive individual of the species;for in the struggle for life to which every animal is exposed, each wouldhave a better chance of supporting itself, by less nutriment being wasted.

Thus, as I believe, natural selection will tend in the long run to reduceany part of the organisation, as soon as it becomes, through changedhabits, superfluous, without by any means causing some other part to belargely developed in a corresponding degree. And conversely, that naturalselection may perfectly well succeed in largely developing an organ withoutrequiring as a necessary compensation the reduction of some adjoining part.

MULTIPLE, RUDIMENTARY, AND LOWLY-ORGANISED STRUCTURES ARE VARIABLE.

It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both withvarieties and species, that when any part or organ is repeated many timesin the same individual (as the vertebrae in snakes, and the stamens inpolyandrous flowers) the number is variable; whereas the number of the samepart or organ, when it occurs in lesser numbers, is constant. The sameauthor as well as some botanists, have further remarked that multiple partsare extremely liable to vary in structure. As "vegetative repetition," touse Professor Owen's expression, is a sign of low organisation; theforegoing statements accord with the common opinion of naturalists, thatbeings which stand low in the scale of nature are more variable than thosewhich are higher. I presume that lowness here means that the several partsof the organisation have been but little specialised for particularfunctions; and as long as the same part has to perform diversified work, wecan perhaps see why it should remain variable, that is, why naturalselection should not have preserved or rejected each little deviation ofform so carefully as when the part has to serve for some one specialpurpose. In the same way that a knife which has to cut all sorts of thingsmay be of almost any shape; whilst a tool for some particular purpose mustbe of some particular shape. Natural selection, it should never beforgotten, can act solely through and for the advantage of each being.

Rudimentary parts, as is generally admitted, are apt to be highly variable. We shall have to recur to this subject; and I will here only add that theirvariability seems to result from their uselessness, and consequently fromnatural selection having had no power to check deviations in theirstructure.

A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER, INCOMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLYVARIABLE.

Several years ago I was much struck by a remark to the above effect made byMr. Waterhouse. Professor Owen, also, seems to have come to a nearlysimilar conclusion. It is hopeless to attempt to convince any one of thetruth of the above proposition without giving the long array of facts whichI have collected, and which cannot possibly be here introduced. I can onlystate my conviction that it is a rule of high generality. I am aware ofseveral causes of error, but I hope that I have made due allowances forthem. It should be understood that the rule by no means applies to anypart, however unusually developed, unless it be unusually developed in onespecies or in a few species in comparison with the same part in manyclosely allied species. Thus, the wing of the bat is a most abnormalstructure in the class of mammals; but the rule would not apply here,because the whole group of bats possesses wings; it would apply only ifsome one species had wings developed in a remarkable manner in comparisonwith the other species of the same genus. The rule applies very stronglyin the case of secondary sexual characters, when displayed in any unusualmanner. The term, secondary sexual characters, used by Hunter, relates tocharacters which are attached to one sex, but are not directly connectedwith the act of reproduction. The rule applies to males and females; butmore rarely to females, as they seldom offer remarkable secondary sexualcharacters. The rule being so plainly applicable in the case of secondarysexual characters, may be due to the great variability of these characters,whether or not displayed in any unusual manner--of which fact I think therecan be little doubt. But that our rule is not confined to secondary sexualcharacters is clearly shown in the case of hermaphrodite cirripedes; Iparticularly attended to Mr. Waterhouse's remark, whilst investigating thisorder, and I am fully convinced that the rule almost always holds good. Ishall, in a future work, give a list of all the more remarkable cases. Iwill here give only one, as it illustrates the rule in its largestapplication. The opercular valves of sessile cirripedes (rock barnacles)are, in every sense of the word, very important structures, and they differextremely little even in distinct genera; but in the several species of onegenus, Pyrgoma, these valves present a marvellous amount ofdiversification; the homologous valves in the different species beingsometimes wholly unlike in shape; and the amount of variation in theindividuals of the same species is so great that it is no exaggeration tostate that the varieties of the same species differ more from each other inthe characters derived from these important organs, than do the speciesbelonging to other distinct genera.

As with birds the individuals of the same species, inhabiting the samecountry, vary extremely little, I have particularly attended to them; andthe rule certainly seems to hold good in this class. I cannot make outthat it applies to plants, and this would have seriously shaken my beliefin its truth, had not the great variability in plants made it particularlydifficult to compare their relative degrees of variability.

When we see any part or organ developed in a remarkable degree or manner ina species, the fair presumption is that it is of high importance to thatspecies: nevertheless it is in this case eminently liable to variation. Why should this be so? On the view that each species has beenindependently created, with all its parts as we now see them, I can see noexplanation. But on the view that groups of species are descended fromsome other species, and have been modified through natural selection, Ithink we can obtain some light. First let me make some preliminaryremarks. If, in our domestic animals, any part or the whole animal beneglected, and no selection be applied, that part (for instance, the combin the Dorking fowl) or the whole breed will cease to have a uniformcharacter: and the breed may be said to be degenerating. In rudimentaryorgans, and in those which have been but little specialised for anyparticular purpose, and perhaps in polymorphic groups, we see a nearlyparallel case; for in such cases natural selection either has not or cannotcome into full play, and thus the organisation is left in a fluctuatingcondition. But what here more particularly concerns us is, that thosepoints in our domestic animals, which at the present time are undergoingrapid change by continued selection, are also eminently liable tovariation. Look at the individuals of the same breed of the pigeon; andsee what a prodigious amount of difference there is in the beak oftumblers, in the beak and wattle of carriers, in the carriage and tail offantails, etc., these being the points now mainly attended to by Englishfanciers. Even in the same sub-breed, as in that of the short-facedtumbler, it is notoriously difficult to breed nearly perfect birds, manydeparting widely from the standard. There may truly be said to be aconstant struggle going on between, on the one hand, the tendency toreversion to a less perfect state, as well as an innate tendency to newvariations, and, on the other hand, the power of steady selection to keepthe breed true. In the long run selection gains the day, and we do notexpect to fail so completely as to breed a bird as coarse as a commontumbler pigeon from a good short-faced strain. But as long as selection israpidly going on, much variability in the parts undergoing modification mayalways be expected.

Now let us turn to nature. When a part has been developed in anextraordinary manner in any one species, compared with the other species ofthe same genus, we may conclude that this part has undergone anextraordinary amount of modification since the period when the severalspecies branched off from the common progenitor of the genus. This periodwill seldom be remote in any extreme degree, as species rarely endure formore than one geological period. An extraordinary amount of modificationimplies an unusually large and long-continued amount of variability, whichhas continually been accumulated by natural selection for the benefit ofthe species. But as the variability of the extraordinarily developed partor organ has been so great and long-continued within a period notexcessively remote, we might, as a general rule, still expect to find morevariability in such parts than in other parts of the organisation whichhave remained for a much longer period nearly constant. And this, I amconvinced, is the case. That the struggle between natural selection on theone hand, and the tendency to reversion and variability on the other hand,will in the course of time cease; and that the most abnormally developedorgans may be made constant, I see no reason to doubt. Hence, when anorgan, however abnormal it may be, has been transmitted in approximatelythe same condition to many modified descendants, as in the case of the wingof the bat, it must have existed, according to our theory, for an immenseperiod in nearly the same state; and thus it has come not to be morevariable than any other structure. It is only in those cases in which themodification has been comparatively recent and extraordinarily great thatwe ought to find the GENERATIVE VARIABILITY, as it may be called, stillpresent in a high degree. For in this case the variability will seldom asyet have been fixed by the continued selection of the individuals varyingin the required manner and degree, and by the continued rejection of thosetending to revert to a former and less modified condition.

SPECIFIC CHARACTERS MORE VARIABLE THAN GENERIC CHARACTERS.

The principle discussed under the last heading may be applied to ourpresent subject. It is notorious that specific characters are morevariable than generic. To explain by a simple example what is meant: ifin a large genus of plants some species had blue flowers and some had red,the colour would be only a specific character, and no one would besurprised at one of the blue species varying into red, or conversely; butif all the species had blue flowers, the colour would become a genericcharacter, and its variation would be a more unusual circumstance. I havechosen this example because the explanation which most naturalists wouldadvance is not here applicable, namely, that specific characters are morevariable than generic, because they are taken from parts of lessphysiological importance than those commonly used for classing genera. Ibelieve this explanation is partly, yet only indirectly, true; I shall,however, have to return to this point in the chapter on Classification. Itwould be almost superfluous to adduce evidence in support of the statement,that ordinary specific characters are more variable than generic; but withrespect to important characters, I have repeatedly noticed in works onnatural history, that when an author remarks with surprise that someimportant organ or part, which is generally very constant throughout alarge group of species, DIFFERS considerably in closely-allied species, itis often VARIABLE in the individuals of the same species. And this factshows that a character, which is generally of generic value, when it sinksin value and becomes only of specific value, often becomes variable, thoughits physiological importance may remain the same. Something of the samekind applies to monstrosities: at least Is. Geoffroy St. Hilaireapparently entertains no doubt, that the more an organ normally differs inthe different species of the same group, the more subject it is toanomalies in the individuals.

On the ordinary view of each species having been independently created, whyshould that part of the structure, which differs from the same part inother independently created species of the same genus, be more variablethan those parts which are closely alike in the several species? I do notsee that any explanation can be given. But on the view that species areonly strongly marked and fixed varieties, we might expect often to findthem still continuing to vary in those parts of their structure which havevaried within a moderately recent period, and which have thus come todiffer. Or to state the case in another manner: the points in which allthe species of a genus resemble each other, and in which they differ fromallied genera, are called generic characters; and these characters may beattributed to inheritance from a common progenitor, for it can rarely havehappened that natural selection will have modified several distinctspecies, fitted to more or less widely different habits, in exactly thesame manner: and as these so-called generic characters have been inheritedfrom before the period when the several species first branched off fromtheir common progenitor, and subsequently have not varied or come to differin any degree, or only in a slight degree, it is not probable that theyshould vary at the present day. On the other hand, the points in whichspecies differ from other species of the same genus are called specificcharacters; and as these specific characters have varied and come to differsince the period when the species branched off from a common progenitor, itis probable that they should still often be in some degree variable--atleast more variable than those parts of the organisation which have for avery long period remained constant.

SECONDARY SEXUAL CHARACTERS VARIABLE.

I think it will be admitted by naturalists, without my entering on details,that secondary sexual characters are highly variable. It will also beadmitted that species of the same group differ from each other more widelyin their secondary sexual characters, than in other parts of theirorganisation; compare, for instance, the amount of difference between themales of gallinaceous birds, in which secondary sexual characters arestrongly displayed, with the amount of difference between the females. Thecause of the original variability of these characters is not manifest; butwe can see why they should not have been rendered as constant and uniformas others, for they are accumulated by sexual selection, which is lessrigid in its action than ordinary selection, as it does not entail death,but only gives fewer offspring to the less favoured males. Whatever thecause may be of the variability of secondary sexual characters, as they arehighly variable, sexual selection will have had a wide scope for action,and may thus have succeeded in giving to the species of the same group agreater amount of difference in these than in other respects.

It is a remarkable fact, that the secondary differences between the twosexes of the same species are generally displayed in the very same parts ofthe organisation in which the species of the same genus differ from eachother. Of this fact I will give in illustration the first two instanceswhich happen to stand on my list; and as the differences in these cases areof a very unusual nature, the relation can hardly be accidental. The samenumber of joints in the tarsi is a character common to very large groups ofbeetles, but in the Engidae, as Westwood has remarked, the number variesgreatly and the number likewise differs in the two sexes of the samespecies. Again in the fossorial hymenoptera, the neuration of the wings isa character of the highest importance, because common to large groups; butin certain genera the neuration differs in the different species, andlikewise in the two sexes of the same species. Sir J. Lubbock has recentlyremarked, that several minute crustaceans offer excellent illustrations ofthis law. "In Pontella, for instance, the sexual characters are affordedmainly by the anterior antennae and by the fifth pair of legs: thespecific differences also are principally given by these organs." Thisrelation has a clear meaning on my view: I look at all the species of thesame genus as having as certainly descended from the same progenitor, ashave the two sexes of any one species. Consequently, whatever part of thestructure of the common progenitor, or of its early descendants, becamevariable; variations of this part would, it is highly probable, be takenadvantage of by natural and sexual selection, in order to fit the severalplaces in the economy of nature, and likewise to fit the two sexes of thesame species to each other, or to fit the males to struggle with othermales for the possession of the females.

Finally, then, I conclude that the greater variability of specificcharacters, or those which distinguish species from species, than ofgeneric characters, or those which are possessed by all the species; thatthe frequent extreme variability of any part which is developed in aspecies in an extraordinary manner in comparison with the same part in itscongeners; and the slight degree of variability in a part, howeverextraordinarily it may be developed, if it be common to a whole group ofspecies; that the great variability of secondary sexual characters andtheir great difference in closely allied species; that secondary sexual andordinary specific differences are generally displayed in the same parts ofthe organisation, are all principles closely connected together. All beingmainly due to the species of the same group being the descendants of acommon progenitor, from whom they have inherited much in common, to partswhich have recently and largely varied being more likely still to go onvarying than parts which have long been inherited and have not varied, tonatural selection having more or less completely, according to the lapse oftime, overmastered the tendency to reversion and to further variability, tosexual selection being less rigid than ordinary selection, and tovariations in the same parts having been accumulated by natural and sexualselection, and thus having been adapted for secondary sexual, and forordinary purposes.

DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS, SO THAT A VARIETY OF ONESPECIES OFTEN ASSUMES A CHARACTER PROPER TO AN ALLIED SPECIES, OR REVERTSTO SOME OF THE CHARACTERS OF AN EARLY PROGENITOR.

These propositions will be most readily understood by looking to ourdomestic races. The most distinct breeds of the pigeon, in countrieswidely apart, present sub-varieties with reversed feathers on the head, andwith feathers on the feet, characters not possessed by the aboriginalrock-pigeon; these then are analogous variations in two or more distinctraces. The frequent presence of fourteen or even sixteen tail-feathers inthe pouter may be considered as a variation representing the normalstructure of another race, the fantail. I presume that no one will doubtthat all such analogous variations are due to the several races of thepigeon having inherited from a common parent the same constitution andtendency to variation, when acted on by similar unknown influences. In thevegetable kingdom we have a case of analogous variation, in the enlargedstems, or as commonly called roots, of the Swedish turnip and ruta-baga,plants which several botanists rank as varieties produced by cultivationfrom a common parent: if this be not so, the case will then be one ofanalogous variation in two so-called distinct species; and to these a thirdmay be added, namely, the common turnip. According to the ordinary view ofeach species having been independently created, we should have to attributethis similarity in the enlarged stems of these three plants, not to thevera causa of community of descent, and a consequent tendency to vary in alike manner, but to three separate yet closely related acts of creation. Many similar cases of analogous variation have been observed by Naudin inthe great gourd family, and by various authors in our cereals. Similarcases occurring with insects under natural conditions have lately beendiscussed with much ability by Mr. Walsh, who has grouped them under hislaw of equable variability.

With pigeons, however, we have another case, namely, the occasionalappearance in all the breeds, of slaty-blue birds with two black bars onthe wings, white loins, a bar at the end of the tail, with the outerfeathers externally edged near their bases with white. As all these marksare characteristic of the parent rock-pigeon, I presume that no one willdoubt that this is a case of reversion, and not of a new yet analogousvariation appearing in the several breeds. We may, I think, confidentlycome to this conclusion, because, as we have seen, these coloured marks areeminently liable to appear in the crossed offspring of two distinct anddifferently coloured breeds; and in this case there is nothing in theexternal conditions of life to cause the reappearance of the slaty-blue,with the several marks, beyond the influence of the mere act of crossing onthe laws of inheritance.

No doubt it is a very surprising fact that characters should reappear afterhaving been lost for many, probably for hundreds of generations. But whena breed has been crossed only once by some other breed, the offspringoccasionally show for many generations a tendency to revert in character tothe foreign breed--some say, for a dozen or even a score of generations. After twelve generations, the proportion of blood, to use a commonexpression, from one ancestor, is only 1 in 2048; and yet, as we see, it isgenerally believed that a tendency to reversion is retained by this remnantof foreign blood. In a breed which has not been crossed, but in which BOTHparents have lost some character which their progenitor possessed, thetendency, whether strong or weak, to reproduce the lost character might, aswas formerly remarked, for all that we can see to the contrary, betransmitted for almost any number of generations. When a character whichhas been lost in a breed, reappears after a great number of generations,the most probable hypothesis is, not that one individual suddenly takesafter an ancestor removed by some hundred generations, but that in eachsuccessive generation the character in question has been lying latent, andat last, under unknown favourable conditions, is developed. With thebarb-pigeon, for instance, which very rarely produces a blue bird, it isprobable that there is a latent tendency in each generation to produce blueplumage. The abstract improbability of such a tendency being transmittedthrough a vast number of generations, is not greater than that of quiteuseless or rudimentary organs being similarly transmitted. A mere tendencyto produce a rudiment is indeed sometimes thus inherited.

As all the species of the same genus are supposed to be descended from acommon progenitor, it might be expected that they would occasionally varyin an analogous manner; so that the varieties of two or more species wouldresemble each other, or that a variety of one species would resemble incertain characters another and distinct species, this other species being,according to our view, only a well-marked and permanent variety. Butcharacters exclusively due to analogous variation would probably be of anunimportant nature, for the preservation of all functionally importantcharacters will have been determined through natural selection, inaccordance with the different habits of the species. It might further beexpected that the species of the same genus would occasionally exhibitreversions to long-lost characters. As, however, we do not know the commonancestor of any natural group, we cannot distinguish between reversionaryand analogous characters. If, for instance, we did not know that theparent rock-pigeon was not feather-footed or turn-crowned, we could nothave told, whether such characters in our domestic breeds were reversionsor only analogous variations; but we might have inferred that the bluecolour was a case of reversion from the number of the markings, which arecorrelated with this tint, and which would not probably have all appearedtogether from simple variation. More especially we might have inferredthis from the blue colour and the several marks so often appearing whendifferently coloured breeds are crossed. Hence, although under nature itmust generally be left doubtful, what cases are reversions to formerlyexisting characters, and what are new but analogous variations, yet weought, on our theory, sometimes to find the varying offspring of a speciesassuming characters which are already present in other members of the samegroup. And this undoubtedly is the case.

The difficulty in distinguishing variable species is largely due to thevarieties mocking, as it were, other species of the same genus. Aconsiderable catalogue, also, could be given of forms intermediate betweentwo other forms, which themselves can only doubtfully be ranked as species;and this shows, unless all these closely allied forms be considered asindependently created species, that they have in varying assumed some ofthe characters of the others. But the best evidence of analogousvariations is afforded by parts or organs which are generally constant incharacter, but which occasionally vary so as to resemble, in some degree,the same part or organ in an allied species. I have collected a long listof such cases; but here, as before, I lie under the great disadvantage ofnot being able to give them. I can only repeat that such cases certainlyoccur, and seem to me very remarkable.

I will, however, give one curious and complex case, not indeed as affectingany important character, but from occurring in several species of the samegenus, partly under domestication and partly under nature. It is a casealmost certainly of reversion. The ass sometimes has very distincttransverse bars on its legs, like those on the legs of a zebra. It hasbeen asserted that these are plainest in the foal, and from inquiries whichI have made, I believe this to be true. The stripe on the shoulder issometimes double, and is very variable in length and outline. A white ass,but NOT an albino, has been described without either spinal or shoulderstripe; and these stripes are sometimes very obscure, or actually quitelost, in dark-coloured asses. The koulan of Pallas is said to have beenseen with a double shoulder-stripe. Mr. Blyth has seen a specimen of thehemionus with a distinct shoulder-stripe, though it properly has none; andI have been informed by Colonel Poole that foals of this species aregenerally striped on the legs and faintly on the shoulder. The quagga,though so plainly barred like a zebra over the body, is without bars on thelegs; but Dr. Gray has figured one specimen with very distinct zebra-likebars on the hocks.

With respect to the horse, I have collected cases in England of the spinalstripe in horses of the most distinct breeds, and of ALL colours;transverse bars on the legs are not rare in duns, mouse-duns, and in oneinstance in a chestnut; a faint shoulder-stripe may sometimes be seen induns, and I have seen a trace in a bay horse. My son made a carefulexamination and sketch for me of a dun Belgian cart-horse with a doublestripe on each shoulder and with leg-stripes. I have myself seen a dunDevonshire pony, and a small dun Welsh pony has been carefully described tome, both with THREE parallel stripes on each shoulder.

In the northwest part of India the Kattywar breed of horses is so generallystriped, that, as I hear from Colonel Poole, who examined this breed forthe Indian Government, a horse without stripes is not considered as purelybred. The spine is always striped; the legs are generally barred; and theshoulder-stripe, which is sometimes double and sometimes treble, is common;the side of the face, moreover, is sometimes striped. The stripes areoften plainest in the foal; and sometimes quite disappear in old horses. Colonel Poole has seen both gray and bay Kattywar horses striped when firstfoaled. I have also reason to suspect, from information given me by Mr.W.W. Edwards, that with the English race-horse the spinal stripe is muchcommoner in the foal than in the full-grown animal. I have myself recentlybred a foal from a bay mare (offspring of a Turkoman horse and a Flemishmare) by a bay English race-horse. This foal, when a week old, was markedon its hinder quarters and on its forehead with numerous very narrow, dark,zebra-like bars, and its legs were feebly striped. All the stripes soondisappeared completely. Without here entering on further details I maystate that I have collected cases of leg and shoulder stripes in horses ofvery different breeds in various countries from Britain to Eastern China;and from Norway in the north to the Malay Archipelago in the south. In allparts of the world these stripes occur far oftenest in duns and mouse-duns;by the term dun a large range of colour is included, from one between brownand black to a close approach to cream colour.

I am aware that Colonel Hamilton Smith, who has written on this subject,believes that the several breeds of the horse are descended from severalaboriginal species, one of which, the dun, was striped; and that theabove-described appearances are all due to ancient crosses with the dunstock. But this view may be safely rejected, for it is highly improbablethat the heavy Belgian cart-horse, Welsh ponies, Norwegian cobs, the lankyKattywar race, etc., inhabiting the most distant parts of the world, shouldhave all have been crossed with one supposed aboriginal stock.

Now let us turn to the effects of crossing the several species of the horsegenus. Rollin asserts that the common mule from the ass and horse isparticularly apt to have bars on its legs; according to Mr. Gosse, incertain parts of the United States, about nine out of ten mules havestriped legs. I once saw a mule with its legs so much striped that any onemight have thought that it was a hybrid zebra; and Mr. W.C. Martin, in hisexcellent treatise on the horse, has given a figure of a similar mule. Infour coloured drawings, which I have seen, of hybrids between the ass andzebra, the legs were much more plainly barred than the rest of the body;and in one of them there was a double shoulder-stripe. In Lord Morton'sfamous hybrid, from a chestnut mare and male quagga, the hybrid and eventhe pure offspring subsequently produced from the same mare by a blackArabian sire, were much more plainly barred across the legs than is eventhe pure quagga. Lastly, and this is another most remarkable case, ahybrid has been figured by Dr. Gray (and he informs me that he knows of asecond case) from the ass and the hemionus; and this hybrid, though the assonly occasionally has stripes on his legs and the hemionus has none and hasnot even a shoulder-stripe, nevertheless had all four legs barred, and hadthree short shoulder-stripes, like those on the dun Devonshire and Welshponies, and even had some zebra-like stripes on the sides of its face. With respect to this last fact, I was so convinced that not even a stripeof colour appears from what is commonly called chance, that I was ledsolely from the occurrence of the face-stripes on this hybrid from the assand hemionus to ask Colonel Poole whether such face-stripes ever occurredin the eminently striped Kattywar breed of horses, and was, as we haveseen, answered in the affirmative.

What now are we to say to these several facts? We see several distinctspecies of the horse genus becoming, by simple variation, striped on thelegs like a zebra, or striped on the shoulders like an ass. In the horsewe see this tendency strong whenever a dun tint appears--a tint whichapproaches to that of the general colouring of the other species of thegenus. The appearance of the stripes is not accompanied by any change ofform, or by any other new character. We see this tendency to becomestriped most strongly displayed in hybrids from between several of the mostdistinct species. Now observe the case of the several breeds of pigeons: they are descended from a pigeon (including two or three sub-species orgeographical races) of a bluish colour, with certain bars and other marks;and when any breed assumes by simple variation a bluish tint, these barsand other marks invariably reappear; but without any other change of formor character. When the oldest and truest breeds of various colours arecrossed, we see a strong tendency for the blue tint and bars and marks toreappear in the mongrels. I have stated that the most probable hypothesisto account for the reappearance of very ancient characters, is--that thereis a TENDENCY in the young of each successive generation to produce thelong-lost character, and that this tendency, from unknown causes, sometimesprevails. And we have just seen that in several species of the horse genusthe stripes are either plainer or appear more commonly in the young than inthe old. Call the breeds of pigeons, some of which have bred true forcenturies, species; and how exactly parallel is the case with that of thespecies of the horse genus! For myself, I venture confidently to look backthousands on thousands of generations, and I see an animal striped like azebra, but perhaps otherwise very differently constructed, the commonparent of our domestic horse (whether or not it be descended from one ormore wild stocks) of the ass, the hemionus, quagga, and zebra.

He who believes that each equine species was independently created, will, Ipresume, assert that each species has been created with a tendency to vary,both under nature and under domestication, in this particular manner, so asoften to become striped like the other species of the genus; and that eachhas been created with a strong tendency, when crossed with speciesinhabiting distant quarters of the world, to produce hybrids resembling intheir stripes, not their own parents, but other species of the genus. Toadmit this view is, as it seems to me, to reject a real for an unreal, orat least for an unknown cause. It makes the works of God a mere mockeryand deception; I would almost as soon believe with the old and ignorantcosmogonists, that fossil shells had never lived, but had been created instone so as to mock the shells now living on the sea-shore.

SUMMARY.

Our ignorance of the laws of variation is profound. Not in one case out ofa hundred can we pretend to assign any reason why this or that part hasvaried. But whenever we have the means of instituting a comparison, thesame laws appear to have acted in producing the lesser differences betweenvarieties of the same species, and the greater differences between speciesof the same genus. Changed conditions generally induce mere fluctuatingvariability, but sometimes they cause direct and definite effects; andthese may become strongly marked in the course of time, though we have notsufficient evidence on this head. Habit in producing constitutionalpeculiarities, and use in strengthening, and disuse in weakening anddiminishing organs, appear in many cases to have been potent in theireffects. Homologous parts tend to vary in the same manner, and homologousparts tend to cohere. Modifications in hard parts and in external partssometimes affect softer and internal parts. When one part is largelydeveloped, perhaps it tends to draw nourishment from the adjoining parts;and every part of the structure which can be saved without detriment willbe saved. Changes of structure at an early age may affect partssubsequently developed; and many cases of correlated variation, the natureof which we are unable to understand, undoubtedly occur. Multiple partsare variable in number and in structure, perhaps arising from such partsnot having been closely specialised for any particular function, so thattheir modifications have not been closely checked by natural selection. Itfollows probably from this same cause, that organic beings low in the scaleare more variable than those standing higher in the scale, and which havetheir whole organisation more specialised. Rudimentary organs, from beinguseless, are not regulated by natural selection, and hence are variable. Specific characters--that is, the characters which have come to differsince the several species of the same genus branched off from a commonparent--are more variable than generic characters, or those which have longbeen inherited, and have not differed within this same period. In theseremarks we have referred to special parts or organs being still variable,because they have recently varied and thus come to differ; but we have alsoseen in the second chapter that the same principle applies to the wholeindividual; for in a district where many species of a genus are found--thatis, where there has been much former variation and differentiation, orwhere the manufactory of new specific forms has been actively at work--inthat district and among these species, we now find, on an average, mostvarieties. Secondary sexual characters are highly variable, and suchcharacters differ much in the species of the same group. Variability inthe same parts of the organisation has generally been taken advantage of ingiving secondary sexual differences to the two sexes of the same species,and specific differences to the several species of the same genus. Anypart or organ developed to an extraordinary size or in an extraordinarymanner, in comparison with the same part or organ in the allied species,must have gone through an extraordinary amount of modification since thegenus arose; and thus we can understand why it should often still bevariable in a much higher degree than other parts; for variation is along-continued and slow process, and natural selection will in such casesnot as yet have had time to overcome the tendency to further variabilityand to reversion to a less modified state. But when a species with anextraordinarily developed organ has become the parent of many modifieddescendants--which on our view must be a very slow process, requiring along lapse of time--in this case, natural selection has succeeded in givinga fixed character to the organ, in however extraordinary a manner it mayhave been developed. Species inheriting nearly the same constitution froma common parent, and exposed to similar influences, naturally tend topresent analogous variations, or these same species may occasionally revertto some of the characters of their ancient progenitors. Although new andimportant modifications may not arise from reversion and analogousvariation, such modifications will add to the beautiful and harmoniousdiversity of nature.

Whatever the cause may be of each slight difference between the offspringand their parents--and a cause for each must exist--we have reason tobelieve that it is the steady accumulation of beneficial differences whichhas given rise to all the more important modifications of structure inrelation to the habits of each species.