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Distribution of fresh-water productions -- On the inhabitants of oceanicislands -- Absence of Batrachians and of terrestrial Mammals -- On therelation of the inhabitants of islands to those of the nearest mainland --On colonisation from the nearest source with subsequent modification --Summary of the last and present chapters.

FRESH-WATER PRODUCTIONS.

As lakes and river-systems are separated from each other by barriers ofland, it might have been thought that fresh-water productions would nothave ranged widely within the same country, and as the sea is apparently astill more formidable barrier, that they would never have extended todistant countries. But the case is exactly the reverse. Not only havemany fresh-water species, belonging to different classes, an enormousrange, but allied species prevail in a remarkable manner throughout theworld. When first collecting in the fresh waters of Brazil, I wellremember feeling much surprise at the similarity of the fresh-waterinsects, shells, etc., and at the dissimilarity of the surroundingterrestrial beings, compared with those of Britain.

But the wide ranging power of fresh-water productions can, I think, in mostcases be explained by their having become fitted, in a manner highly usefulto them, for short and frequent migrations from pond to pond, or fromstream to stream, within their own countries; and liability to widedispersal would follow from this capacity as an almost necessaryconsequence. We can here consider only a few cases; of these, some of themost difficult to explain are presented by fish. It was formerly believedthat the same fresh-water species never existed on two continents distantfrom each other. But Dr. Gunther has lately shown that the Galaxiasattenuatus inhabits Tasmania, New Zealand, the Falkland Islands and themainland of South America. This is a wonderful case, and probablyindicates dispersal from an Antarctic centre during a former warm period. This case, however, is rendered in some degree less surprising by thespecies of this genus having the power of crossing by some unknown meansconsiderable spaces of open ocean: thus there is one species common to NewZealand and to the Auckland Islands, though separated by a distance ofabout 230 miles. On the same continent fresh-water fish often rangewidely, and as if capriciously; for in two adjoining river systems some ofthe species may be the same and some wholly different.

It is probable that they are occasionally transported by what may be calledaccidental means. Thus fishes still alive are not very rarely dropped atdistant points by whirlwinds; and it is known that the ova retain theirvitality for a considerable time after removal from the water. Theirdispersal may, however, be mainly attributed to changes in the level of theland within the recent period, causing rivers to flow into each other. Instances, also, could be given of this having occurred during floods,without any change of level. The wide differences of the fish on theopposite sides of most mountain-ranges, which are continuous andconsequently must, from an early period, have completely prevented theinosculation of the river systems on the two sides, leads to the sameconclusion. Some fresh-water fish belong to very ancient forms, and insuch cases there will have been ample time for great geographical changes,and consequently time and means for much migration. Moreover, Dr. Guntherhas recently been led by several considerations to infer that with fishesthe same forms have a long endurance. Salt-water fish can with care beslowly accustomed to live in fresh water; and, according to Valenciennes,there is hardly a single group of which all the members are confined tofresh water, so that a marine species belonging to a fresh-water groupmight travel far along the shores of the sea, and could, it is probable,become adapted without much difficulty to the fresh waters of a distantland.

Some species of fresh-water shells have very wide ranges, and alliedspecies which, on our theory, are descended from a common parent, and musthave proceeded from a single source, prevail throughout the world. Theirdistribution at first perplexed me much, as their ova are not likely to betransported by birds; and the ova, as well as the adults, are immediatelykilled by sea-water. I could not even understand how some naturalisedspecies have spread rapidly throughout the same country. But two facts,which I have observed--and many others no doubt will be discovered--throwsome light on this subject. When ducks suddenly emerge from a pond coveredwith duck-weed, I have twice seen these little plants adhering to theirbacks; and it has happened to me, in removing a little duck-weed from oneaquarium to another, that I have unintentionally stocked the one withfresh-water shells from the other. But another agency is perhaps moreeffectual: I suspended the feet of a duck in an aquarium, where many ovaof fresh-water shells were hatching; and I found that numbers of theextremely minute and just-hatched shells crawled on the feet, and clung tothem so firmly that when taken out of the water they could not be jarredoff, though at a somewhat more advanced age they would voluntarily dropoff. These just-hatched molluscs, though aquatic in their nature, survivedon the duck's feet, in damp air, from twelve to twenty hours; and in thislength of time a duck or heron might fly at least six or seven hundredmiles, and if blown across the sea to an oceanic island, or to any otherdistant point, would be sure to alight on a pool or rivulet. Sir CharlesLyell informs me that a Dyticus has been caught with an Ancylus (afresh-water shell like a limpet) firmly adhering to it; and a water-beetleof the same family, a Colymbetes, once flew on board the "Beagle," whenforty-five miles distant from the nearest land: how much farther it mighthave been blown by a favouring gale no one can tell.

With respect to plants, it has long been known what enormous ranges manyfresh-water, and even marsh-species, have, both over continents and to themost remote oceanic islands. This is strikingly illustrated, according toAlph. de Candolle, in those large groups of terrestrial plants, which havevery few aquatic members; for the latter seem immediately to acquire, as ifin consequence, a wide range. I think favourable means of dispersalexplain this fact. I have before mentioned that earth occasionally adheresin some quantity to the feet and beaks of birds. Wading birds, whichfrequent the muddy edges of ponds, if suddenly flushed, would be the mostlikely to have muddy feet. Birds of this order wander more than those ofany other; and are occasionally found on the most remote and barren islandsof the open ocean; they would not be likely to alight on the surface of thesea, so that any dirt on their feet would not be washed off; and whengaining the land, they would be sure to fly to their natural fresh-waterhaunts. I do not believe that botanists are aware how charged the mud ofponds is with seeds: I have tried several little experiments, but willhere give only the most striking case: I took in February threetablespoonfuls of mud from three different points, beneath water, on theedge of a little pond; this mud when dry weighed only 6 and 3/4 ounces; Ikept it covered up in my study for six months, pulling up and counting eachplant as it grew; the plants were of many kinds, and were altogether 537 innumber; and yet the viscid mud was all contained in a breakfast cup! Considering these facts, I think it would be an inexplicable circumstanceif water-birds did not transport the seeds of fresh-water plants tounstocked ponds and streams, situated at very distant points. The sameagency may have come into play with the eggs of some of the smallerfresh-water animals.

Other and unknown agencies probably have also played a part. I have statedthat fresh-water fish eat some kinds of seeds, though they reject manyother kinds after having swallowed them; even small fish swallow seeds ofmoderate size, as of the yellow water-lily and Potamogeton. Herons andother birds, century after century, have gone on daily devouring fish; theythen take flight and go to other waters, or are blown across the sea; andwe have seen that seeds retain their power of germination, when rejectedmany hours afterwards in pellets or in the excrement. When I saw the greatsize of the seeds of that fine water-lily, the Nelumbium, and rememberedAlph. de Candolle's remarks on the distribution of this plant, I thoughtthat the means of its dispersal must remain inexplicable; but Audubonstates that he found the seeds of the great southern water-lily (probablyaccording to Dr. Hooker, the Nelumbium luteum) in a heron's stomach. Nowthis bird must often have flown with its stomach thus well stocked todistant ponds, and, then getting a hearty meal of fish, analogy makes mebelieve that it would have rejected the seeds in the pellet in a fit statefor germination.

In considering these several means of distribution, it should be rememberedthat when a pond or stream is first formed, for instance on a rising islet,it will be unoccupied; and a single seed or egg will have a good chance ofsucceeding. Although there will always be a struggle for life between theinhabitants of the same pond, however few in kind, yet as the number evenin a well-stocked pond is small in comparison with the number of speciesinhabiting an equal area of land, the competition between them willprobably be less severe than between terrestrial species; consequently anintruder from the waters of a foreign country would have a better chance ofseizing on a new place, than in the case of terrestrial colonists. Weshould also remember that many fresh-water productions are low in the scaleof nature, and we have reason to believe that such beings become modifiedmore slowly than the high; and this will give time for the migration ofaquatic species. We should not forget the probability of many fresh-waterforms having formerly ranged continuously over immense areas, and thenhaving become extinct at intermediate points. But the wide distribution offresh-water plants, and of the lower animals, whether retaining the sameidentical form, or in some degree modified, apparently depends in main parton the wide dispersal of their seeds and eggs by animals, more especiallyby fresh-water birds, which have great powers of flight, and naturallytravel from one piece of water to another.

ON THE INHABITANTS OF OCEANIC ISLANDS.

We now come to the last of the three classes of facts, which I haveselected as presenting the greatest amount of difficulty with respect todistribution, on the view that not only all the individuals of the samespecies have migrated from some one area, but that allied species, althoughnow inhabiting the most distant points, have proceeded from a single area,the birthplace of their early progenitors. I have already given my reasonsfor disbelieving in continental extensions within the period of existingspecies on so enormous a scale that all the many islands of the severaloceans were thus stocked with their present terrestrial inhabitants. Thisview removes many difficulties, but it does not accord with all the factsin regard to the productions of islands. In the following remarks I shallnot confine myself to the mere question of dispersal, but shall considersome other cases bearing on the truth of the two theories of independentcreation and of descent with modification.

The species of all kinds which inhabit oceanic islands are few in numbercompared with those on equal continental areas: Alph. de Candolle admitsthis for plants, and Wollaston for insects. New Zealand, for instance,with its lofty mountains and diversified stations, extending over 780 milesof latitude, together with the outlying islands of Auckland, Campbell andChatham, contain altogether only 960 kinds of flowering plants; if wecompare this moderate number with the species which swarm over equal areasin Southwestern Australia or at the Cape of Good Hope, we must admit thatsome cause, independently of different physical conditions, has given riseto so great a difference in number. Even the uniform county of Cambridgehas 847 plants, and the little island of Anglesea 764, but a few ferns anda few introduced plants are included in these numbers, and the comparisonin some other respects is not quite fair. We have evidence that the barrenisland of Ascension aboriginally possessed less than half-a-dozen floweringplants; yet many species have now become naturalised on it, as they have inNew Zealand and on every other oceanic island which can be named. In St.Helena there is reason to believe that the naturalised plants and animalshave nearly or quite exterminated many native productions. He who admitsthe doctrine of the creation of each separate species, will have to admitthat a sufficient number of the best adapted plants and animals were notcreated for oceanic islands; for man has unintentionally stocked them farmore fully and perfectly than did nature.

Although in oceanic islands the species are few in number, the proportionof endemic kinds (i.e. those found nowhere else in the world) is oftenextremely large. If we compare, for instance, the number of endemicland-shells in Madeira, or of endemic birds in the Galapagos Archipelago,with the number found on any continent, and then compare the area of theisland with that of the continent, we shall see that this is true. Thisfact might have been theoretically expected, for, as already explained,species occasionally arriving, after long intervals of time in the new andisolated district, and having to compete with new associates, would beeminently liable to modification, and would often produce groups ofmodified descendants. But it by no means follows that, because in anisland nearly all the species of one class are peculiar, those of anotherclass, or of another section of the same class, are peculiar; and thisdifference seems to depend partly on the species which are not modifiedhaving immigrated in a body, so that their mutual relations have not beenmuch disturbed; and partly on the frequent arrival of unmodified immigrantsfrom the mother-country, with which the insular forms have intercrossed. It should be borne in mind that the offspring of such crosses wouldcertainly gain in vigour; so that even an occasional cross would producemore effect than might have been anticipated. I will give a fewillustrations of the foregoing remarks: in the Galapagos Islands there aretwenty-six land birds; of these twenty-one (or perhaps twenty-three) arepeculiar; whereas of the eleven marine birds only two are peculiar; and itis obvious that marine birds could arrive at these islands much more easilyand frequently than land-birds. Bermuda, on the other hand, which lies atabout the same distance from North America as the Galapagos Islands do fromSouth America, and which has a very peculiar soil, does not possess asingle endemic land bird; and we know from Mr. J.M. Jones's admirableaccount of Bermuda, that very many North American birds occasionally oreven frequently visit this island. Almost every year, as I am informed byMr. E.V. Harcourt, many European and African birds are blown to Madeira;this island is inhabited by ninety-nine kinds, of which one alone ispeculiar, though very closely related to a European form; and three or fourother species are confined to this island and to the Canaries. So that theislands of Bermuda and Madeira have been stocked from the neighbouringcontinents with birds, which for long ages have there struggled together,and have become mutually co-adapted. Hence, when settled in their newhomes, each kind will have been kept by the others to its proper place andhabits, and will consequently have been but little liable to modification. Any tendency to modification will also have been checked by intercrossingwith the unmodified immigrants, often arriving from the mother-country. Madeira again is inhabited by a wonderful number of peculiar land-shells,whereas not one species of sea-shell is peculiar to its shores: now,though we do not know how sea-shells are dispersed, yet we can see thattheir eggs or larvae, perhaps attached to seaweed or floating timber, or tothe feet of wading birds, might be transported across three or four hundredmiles of open sea far more easily than land-shells. The different ordersof insects inhabiting Madeira present nearly parallel cases.

Oceanic islands are sometimes deficient in animals of certain wholeclasses, and their places are occupied by other classes; thus in theGalapagos Islands reptiles, and in New Zealand gigantic wingless birds,take, or recently took, the place of mammals. Although New Zealand is herespoken of as an oceanic island, it is in some degree doubtful whether itshould be so ranked; it is of large size, and is not separated fromAustralia by a profoundly deep sea; from its geological character and thedirection of its mountain ranges, the Rev. W.B. Clarke has latelymaintained that this island, as well as New Caledonia, should be consideredas appurtenances of Australia. Turning to plants, Dr. Hooker has shownthat in the Galapagos Islands the proportional numbers of the differentorders are very different from what they are elsewhere. All suchdifferences in number, and the absence of certain whole groups of animalsand plants, are generally accounted for by supposed differences in thephysical conditions of the islands; but this explanation is not a littledoubtful. Facility of immigration seems to have been fully as important asthe nature of the conditions.

Many remarkable little facts could be given with respect to the inhabitantsof oceanic islands. For instance, in certain islands not tenanted by asingle mammal, some of the endemic plants have beautifully hooked seeds;yet few relations are more manifest than that hooks serve for thetransportal of seeds in the wool or fur of quadrupeds. But a hooked seedmight be carried to an island by other means; and the plant then becomingmodified would form an endemic species, still retaining its hooks, whichwould form a useless appendage, like the shrivelled wings under thesoldered wing-covers of many insular beetles. Again, islands often possesstrees or bushes belonging to orders which elsewhere include only herbaceousspecies; now trees, as Alph. de Candolle has shown, generally have,whatever the cause may be, confined ranges. Hence trees would be littlelikely to reach distant oceanic islands; and an herbaceous plant, which hadno chance of successfully competing with the many fully developed treesgrowing on a continent, might, when established on an island, gain anadvantage over other herbaceous plants by growing taller and taller andovertopping them. In this case, natural selection would tend to add to thestature of the plant, to whatever order it belonged, and thus first convertit into a bush and then into a tree.

ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON OCEANIC ISLANDS.

With respect to the absence of whole orders of animals on oceanic islands,Bory St. Vincent long ago remarked that Batrachians (frogs, toads, newts)are never found on any of the many islands with which the great oceans arestudded. I have taken pains to verify this assertion, and have found ittrue, with the exception of New Zealand, New Caledonia, the AndamanIslands, and perhaps the Solomon Islands and the Seychelles. But I havealready remarked that it is doubtful whether New Zealand and New Caledoniaought to be classed as oceanic islands; and this is still more doubtfulwith respect to the Andaman and Solomon groups and the Seychelles. Thisgeneral absence of frogs, toads and newts on so many true oceanic islandscannot be accounted for by their physical conditions; indeed it seems thatislands are peculiarly fitted for these animals; for frogs have beenintroduced into Madeira, the Azores, and Mauritius, and have multiplied soas to become a nuisance. But as these animals and their spawn areimmediately killed (with the exception, as far as known, of one Indianspecies) by sea-water, there would be great difficulty in their transportalacross the sea, and therefore we can see why they do not exist on strictlyoceanic islands. But why, on the theory of creation, they should not havebeen created there, it would be very difficult to explain.

Mammals offer another and similar case. I have carefully searched theoldest voyages, and have not found a single instance, free from doubt, of aterrestrial mammal (excluding domesticated animals kept by the natives)inhabiting an island situated above 300 miles from a continent or greatcontinental island; and many islands situated at a much less distance areequally barren. The Falkland Islands, which are inhabited by a wolf-likefox, come nearest to an exception; but this group cannot be considered asoceanic, as it lies on a bank in connection with the mainland at a distanceof about 280 miles; moreover, icebergs formerly brought boulders to itswestern shores, and they may have formerly transported foxes, as nowfrequently happens in the arctic regions. Yet it cannot be said that smallislands will not support at least small mammals, for they occur in manyparts of the world on very small islands, when lying close to a continent;and hardly an island can be named on which our smaller quadrupeds have notbecome naturalised and greatly multiplied. It cannot be said, on theordinary view of creation, that there has not been time for the creation ofmammals; many volcanic islands are sufficiently ancient, as shown by thestupendous degradation which they have suffered, and by their tertiarystrata: there has also been time for the production of endemic speciesbelonging to other classes; and on continents it is known that new speciesof mammals appear and disappear at a quicker rate than other and loweranimals. Although terrestrial mammals do not occur on oceanic islands,aerial mammals do occur on almost every island. New Zealand possesses twobats found nowhere else in the world: Norfolk Island, the VitiArchipelago, the Bonin Islands, the Caroline and Marianne Archipelagoes,and Mauritius, all possess their peculiar bats. Why, it may be asked, hasthe supposed creative force produced bats and no other mammals on remoteislands? On my view this question can easily be answered; for noterrestrial mammal can be transported across a wide space of sea, but batscan fly across. Bats have been seen wandering by day far over the AtlanticOcean; and two North American species, either regularly or occasionally,visit Bermuda, at the distance of 600 miles from the mainland. I hear fromMr. Tomes, who has specially studied this family, that many species haveenormous ranges, and are found on continents and on far distant islands. Hence, we have only to suppose that such wandering species have beenmodified in their new homes in relation to their new position, and we canunderstand the presence of endemic bats on oceanic islands, with theabsence of all other terrestrial mammals.

Another interesting relation exists, namely, between the depth of the seaseparating islands from each other, or from the nearest continent, and thedegree of affinity of their mammalian inhabitants. Mr. Windsor Earl hasmade some striking observations on this head, since greatly extended by Mr.Wallace's admirable researches, in regard to the great Malay Archipelago,which is traversed near Celebes by a space of deep ocean, and thisseparates two widely distinct mammalian faunas. On either side, theislands stand on a moderately shallow submarine bank, and these islands areinhabited by the same or by closely allied quadrupeds. I have not as yethad time to follow up this subject in all quarters of the world; but as faras I have gone, the relation holds good. For instance, Britain isseparated by a shallow channel from Europe, and the mammals are the same onboth sides; and so it is with all the islands near the shores of Australia. The West Indian Islands, on the other hand, stand on a deeply submergedbank, nearly one thousand fathoms in depth, and here we find Americanforms, but the species and even the genera are quite distinct. As theamount of modification which animals of all kinds undergo partly depends onthe lapse of time, and as the islands which are separated from each other,or from the mainland, by shallow channels, are more likely to have beencontinuously united within a recent period than the islands separated bydeeper channels, we can understand how it is that a relation exists betweenthe depth of the sea separating two mammalian faunas, and the degree oftheir affinity, a relation which is quite inexplicable on the theory ofindependent acts of creation.

The foregoing statements in regard to the inhabitants of oceanic islands,namely, the fewness of the species, with a large proportion consisting ofendemic forms--the members of certain groups, but not those of other groupsin the same class, having been modified--the absence of certain wholeorders, as of batrachians and of terrestrial mammals, notwithstanding thepresence of aerial bats, the singular proportions of certain orders ofplants, herbaceous forms having been developed into trees, etc., seem to meto accord better with the belief in the efficiency of occasional means oftransport, carried on during a long course of time, than with the belief inthe former connection of all oceanic islands with the nearest continent;for on this latter view it is probable that the various classes would haveimmigrated more uniformly, and from the species having entered in a body,their mutual relations would not have been much disturbed, andconsequently, they would either have not been modified, or all the speciesin a more equable manner.

I do not deny that there are many and serious difficulties in understandinghow many of the inhabitants of the more remote islands, whether stillretaining the same specific form or subsequently modified, have reachedtheir present homes. But the probability of other islands having onceexisted as halting-places, of which not a wreck now remains, must not beoverlooked. I will specify one difficult case. Almost all oceanicislands, even the most isolated and smallest, are inhabited by land-shells,generally by endemic species, but sometimes by species found elsewherestriking instances of which have been given by Dr. A.A. Gould in relationto the Pacific. Now it is notorious that land-shells are easily killed bysea-water; their eggs, at least such as I have tried, sink in it and arekilled. Yet there must be some unknown, but occasionally efficient meansfor their transportal. Would the just-hatched young sometimes adhere tothe feet of birds roosting on the ground and thus get transported? Itoccurred to me that land-shells, when hybernating and having a membranousdiaphragm over the mouth of the shell, might be floated in chinks ofdrifted timber across moderately wide arms of the sea. And I find thatseveral species in this state withstand uninjured an immersion in sea-waterduring seven days. One shell, the Helix pomatia, after having been thustreated, and again hybernating, was put into sea-water for twenty days andperfectly recovered. During this length of time the shell might have beencarried by a marine country of average swiftness to a distance of 660geographical miles. As this Helix has a thick calcareous operculum Iremoved it, and when it had formed a new membranous one, I again immersedit for fourteen days in sea-water, and again it recovered and crawled away.Baron Aucapitaine has since tried similar experiments. He placed 100 land-shells, belonging to ten species, in a box pierced with holes, and immersedit for a fortnight in the sea. Out of the hundred shells twenty-sevenrecovered. The presence of an operculum seems to have been of importance,as out of twelve specimens of Cyclostoma elegans, which is thus furnished,eleven revived. It is remarkable, seeing how well the Helix pomatiaresisted with me the salt-water, that not one of fifty-four specimensbelonging to four other species of Helix tried by Aucapitaine recovered. It is, however, not at all probable that land-shells have often been thustransported; the feet of birds offer a more probable method.

ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO THOSE OF THE NEARESTMAINLAND.

The most striking and important fact for us is the affinity of the specieswhich inhabit islands to those of the nearest mainland, without beingactually the same. Numerous instances could be given. The GalapagosArchipelago, situated under the equator, lies at a distance of between 500and 600 miles from the shores of South America. Here almost every productof the land and of the water bears the unmistakable stamp of the Americancontinent. There are twenty-six land birds. Of these twenty-one, orperhaps twenty-three, are ranked as distinct species, and would commonly beassumed to have been here created; yet the close affinity of most of thesebirds to American species is manifest in every character in their habits,gestures, and tones of voice. So it is with the other animals, and with alarge proportion of the plants, as shown by Dr. Hooker in his admirableFlora of this archipelago. The naturalist, looking at the inhabitants ofthese volcanic islands in the Pacific, distant several hundred miles fromthe continent, feels that he is standing on American land. Why should thisbe so? Why should the species which are supposed to have been created inthe Galapagos Archipelago, and nowhere else, bear so plainly the stamp ofaffinity to those created in America? There is nothing in the conditionsof life, in the geological nature of the islands, in their height orclimate, or in the proportions in which the several classes are associatedtogether, which closely resembles the conditions of the South Americancoast. In fact, there is a considerable dissimilarity in all theserespects. On the other hand, there is a considerable degree of resemblancein the volcanic nature of the soil, in the climate, height, and size of theislands, between the Galapagos and Cape Verde Archipelagos: but what anentire and absolute difference in their inhabitants! The inhabitants ofthe Cape Verde Islands are related to those of Africa, like those of theGalapagos to America. Facts, such as these, admit of no sort ofexplanation on the ordinary view of independent creation; whereas, on theview here maintained, it is obvious that the Galapagos Islands would belikely to receive colonists from America, whether by occasional means oftransport or (though I do not believe in this doctrine) by formerlycontinuous land, and the Cape Verde Islands from Africa; such colonistswould be liable to modification--the principle of inheritance stillbetraying their original birthplace.

Many analogous facts could be given: indeed it is an almost universal rulethat the endemic productions of islands are related to those of the nearestcontinent, or of the nearest large island. The exceptions are few, andmost of them can be explained. Thus, although Kerguelen Land stands nearerto Africa than to America, the plants are related, and that very closely,as we know from Dr. Hooker's account, to those of America: but on the viewthat this island has been mainly stocked by seeds brought with earth andstones on icebergs, drifted by the prevailing currents, this anomalydisappears. New Zealand in its endemic plants is much more closely relatedto Australia, the nearest mainland, than to any other region: and this iswhat might have been expected; but it is also plainly related to SouthAmerica, which, although the next nearest continent, is so enormouslyremote, that the fact becomes an anomaly. But this difficulty partiallydisappears on the view that New Zealand, South America, and the othersouthern lands, have been stocked in part from a nearly intermediate thoughdistant point, namely, from the antarctic islands, when they were clothedwith vegetation, during a warmer tertiary period, before the commencementof the last Glacial period. The affinity, which, though feeble, I amassured by Dr. Hooker is real, between the flora of the south-westerncorner of Australia and of the Cape of Good Hope, is a far more remarkablecase; but this affinity is confined to the plants, and will, no doubt, someday be explained.

The same law which has determined the relationship between the inhabitantsof islands and the nearest mainland, is sometimes displayed on a smallscale, but in a most interesting manner, within the limits of the samearchipelago. Thus each separate island of the Galapagos Archipelago istenanted, and the fact is a marvellous one, by many distinct species; butthese species are related to each other in a very much closer manner thanto the inhabitants of the American continent, or of any other quarter ofthe world. This is what might have been expected, for islands situated sonear to each other would almost necessarily receive immigrants from thesame original source, and from each other. But how is it that many of theimmigrants have been differently modified, though only in a small degree,in islands situated within sight of each other, having the same geologicalnature, the same height, climate, etc? This long appeared to me a greatdifficulty: but it arises in chief part from the deeply-seated error ofconsidering the physical conditions of a country as the most important;whereas it cannot be disputed that the nature of the other species withwhich each has to compete, is at least as important, and generally a farmore important element of success. Now if we look to the species whichinhabit the Galapagos Archipelago, and are likewise found in other parts ofthe world, we find that they differ considerably in the several islands. This difference might indeed have been expected if the islands have beenstocked by occasional means of transport--a seed, for instance, of oneplant having been brought to one island, and that of another plant toanother island, though all proceeding from the same general source. Hence,when in former times an immigrant first settled on one of the islands, orwhen it subsequently spread from one to another, it would undoubtedly beexposed to different conditions in the different islands, for it would haveto compete with a different set of organisms; a plant, for instance, wouldfind the ground best-fitted for it occupied by somewhat different speciesin the different islands, and would be exposed to the attacks of somewhatdifferent enemies. If, then, it varied, natural selection would probablyfavour different varieties in the different islands. Some species,however, might spread and yet retain the same character throughout thegroup, just as we see some species spreading widely throughout a continentand remaining the same.

The really surprising fact in this case of the Galapagos Archipelago, andin a lesser degree in some analogous cases, is that each new species afterbeing formed in any one island, did not spread quickly to the otherislands. But the islands, though in sight of each other, are separated bydeep arms of the sea, in most cases wider than the British Channel, andthere is no reason to suppose that they have at any former period beencontinuously united. The currents of the sea are rapid and deep betweenthe islands, and gales of wind are extraordinarily rare; so that theislands are far more effectually separated from each other than they appearon a map. Nevertheless, some of the species, both of those found in otherparts of the world and of those confined to the archipelago, are common tothe several islands; and we may infer from the present manner ofdistribution that they have spread from one island to the others. But weoften take, I think, an erroneous view of the probability of closely alliedspecies invading each other's territory, when put into freeintercommunication. Undoubtedly, if one species has any advantage overanother, it will in a very brief time wholly or in part supplant it; but ifboth are equally well fitted for their own places, both will probably holdtheir separate places for almost any length of time. Being familiar withthe fact that many species, naturalised through man's agency, have spreadwith astonishing rapidity over wide areas, we are apt to infer that mostspecies would thus spread; but we should remember that the species whichbecome naturalised in new countries are not generally closely allied to theaboriginal inhabitants, but are very distinct forms, belonging in a largeproportion of cases, as shown by Alph. de Candolle, to distinct genera. Inthe Galapagos Archipelago, many even of the birds, though so well adaptedfor flying from island to island, differ on the different islands; thusthere are three closely allied species of mocking-thrush, each confined toits own island. Now let us suppose the mocking-thrush of Chatham Island tobe blown to Charles Island, which has its own mocking-thrush; why should itsucceed in establishing itself there? We may safely infer that CharlesIsland is well stocked with its own species, for annually more eggs arelaid and young birds hatched than can possibly be reared; and we may inferthat the mocking-thrush peculiar to Charles Island is at least as wellfitted for its home as is the species peculiar to Chatham Island. Sir C.Lyell and Mr. Wollaston have communicated to me a remarkable fact bearingon this subject; namely, that Madeira and the adjoining islet of PortoSanto possess many distinct but representative species of land-shells, someof which live in crevices of stone; and although large quantities of stoneare annually transported from Porto Santo to Madeira, yet this latterisland has not become colonised by the Porto Santo species: nevertheless,both islands have been colonised by some European land-shells, which nodoubt had some advantage over the indigenous species. From theseconsiderations I think we need not greatly marvel at the endemic specieswhich inhabit the several islands of the Galapagos Archipelago not havingall spread from island to island. On the same continent, also,pre-occupation has probably played an important part in checking thecommingling of the species which inhabit different districts with nearlythe same physical conditions. Thus, the south-east and south-west cornersof Australia have nearly the same physical conditions, and are united bycontinuous land, yet they are inhabited by a vast number of distinctmammals, birds, and plants; so it is, according to Mr. Bates, with thebutterflies and other animals inhabiting the great, open, and continuousvalley of the Amazons.

The same principle which governs the general character of the inhabitantsof oceanic islands, namely, the relation to the source whence colonistscould have been most easily derived, together with their subsequentmodification, is of the widest application throughout nature. We see thison every mountain-summit, in every lake and marsh. For Alpine species,excepting in as far as the same species have become widely spread duringthe Glacial epoch, are related to those of the surrounding lowlands; thuswe have in South America, Alpine humming-birds, Alpine rodents, Alpineplants, etc., all strictly belonging to American forms; and it is obviousthat a mountain, as it became slowly upheaved, would be colonised from thesurrounding lowlands. So it is with the inhabitants of lakes and marshes,excepting in so far as great facility of transport has allowed the sameforms to prevail throughout large portions of the world. We see the sameprinciple in the character of most of the blind animals inhabiting thecaves of America and of Europe. Other analogous facts could be given. Itwill, I believe, be found universally true, that wherever in two regions,let them be ever so distant, many closely allied or representative speciesoccur, there will likewise be found some identical species; and wherevermany closely-allied species occur, there will be found many forms whichsome naturalists rank as distinct species, and others as mere varieties;these doubtful forms showing us the steps in the process of modification.

The relation between the power and extent of migration in certain species,either at the present or at some former period, and the existence at remotepoints of the world of closely allied species, is shown in another and moregeneral way. Mr. Gould remarked to me long ago, that in those genera ofbirds which range over the world, many of the species have very wideranges. I can hardly doubt that this rule is generally true, thoughdifficult of proof. Among mammals, we see it strikingly displayed in Bats,and in a lesser degree in the Felidae and Canidae. We see the same rule inthe distribution of butterflies and beetles. So it is with most of theinhabitants of fresh water, for many of the genera in the most distinctclasses range over the world, and many of the species have enormous ranges. It is not meant that all, but that some of the species have very wideranges in the genera which range very widely. Nor is it meant that thespecies in such genera have, on an average, a very wide range; for thiswill largely depend on how far the process of modification has gone; forinstance, two varieties of the same species inhabit America and Europe, andthus the species has an immense range; but, if variation were to be carrieda little further, the two varieties would be ranked as distinct species,and their range would be greatly reduced. Still less is it meant, thatspecies which have the capacity of crossing barriers and ranging widely, asin the case of certain powerfully-winged birds, will necessarily rangewidely; for we should never forget that to range widely implies not onlythe power of crossing barriers, but the more important power of beingvictorious in distant lands in the struggle for life with foreignassociates. But according to the view that all the species of a genus,though distributed to the most remote points of the world, are descendedfrom a single progenitor, we ought to find, and I believe as a general rulewe do find, that some at least of the species range very widely.

We should bear in mind that many genera in all classes are of ancientorigin, and the species in this case will have had ample time for dispersaland subsequent modification. There is also reason to believe, fromgeological evidence, that within each great class the lower organismschange at a slower rate than the higher; consequently they will have had abetter chance of ranging widely and of still retaining the same specificcharacter. This fact, together with that of the seeds and eggs of mostlowly organised forms being very minute and better fitted for distanttransportal, probably accounts for a law which has long been observed, andwhich has lately been discussed by Alph. de Candolle in regard to plants,namely, that the lower any group of organisms stands the more widely itranges.

The relations just discussed--namely, lower organisms ranging more widelythan the higher--some of the species of widely-ranging genera themselvesranging widely--such facts, as alpine, lacustrine, and marsh productionsbeing generally related to those which live on the surrounding low landsand dry lands--the striking relationship between the inhabitants of islandsand those of the nearest mainland--the still closer relationship of thedistinct inhabitants of the islands of the same archipelago--areinexplicable on the ordinary view of the independent creation of eachspecies, but are explicable if we admit colonisation from the nearest orreadiest source, together with the subsequent adaptation of the coloniststo their new homes.

SUMMARY OF THE LAST AND PRESENT CHAPTERS.

In these chapters I have endeavoured to show that if we make due allowancefor our ignorance of the full effects of changes of climate and of thelevel of the land, which have certainly occurred within the recent period,and of other changes which have probably occurred--if we remember howignorant we are with respect to the many curious means of occasionaltransport--if we bear in mind, and this is a very important consideration,how often a species may have ranged continuously over a wide area, and thenhave become extinct in the intermediate tracts--the difficulty is notinsuperable in believing that all the individuals of the same species,wherever found, are descended from common parents. And we are led to thisconclusion, which has been arrived at by many naturalists under thedesignation of single centres of creation, by various generalconsiderations, more especially from the importance of barriers of allkinds, and from the analogical distribution of subgenera, genera, andfamilies.

With respect to distinct species belonging to the same genus, which on ourtheory have spread from one parent-source; if we make the same allowancesas before for our ignorance, and remember that some forms of life havechanged very slowly, enormous periods of time having been thus granted fortheir migration, the difficulties are far from insuperable; though in thiscase, as in that of the individuals of the same species, they are oftengreat.

As exemplifying the effects of climatical changes on distribution, I haveattempted to show how important a part the last Glacial period has played,which affected even the equatorial regions, and which, during thealternations of the cold in the north and the south, allowed theproductions of opposite hemispheres to mingle, and left some of themstranded on the mountain-summits in all parts of the world. As showing howdiversified are the means of occasional transport, I have discussed at somelittle length the means of dispersal of fresh-water productions.

If the difficulties be not insuperable in admitting that in the long courseof time all the individuals of the same species, and likewise of theseveral species belonging to the same genus, have proceeded from some onesource; then all the grand leading facts of geographical distribution areexplicable on the theory of migration, together with subsequentmodification and the multiplication of new forms. We can thus understandthe high importance of barriers, whether of land or water, in not onlyseparating but in apparently forming the several zoological and botanicalprovinces. We can thus understand the concentration of related specieswithin the same areas; and how it is that under different latitudes, forinstance, in South America, the inhabitants of the plains and mountains, ofthe forests, marshes, and deserts, are linked together in so mysterious amanner, and are likewise linked to the extinct beings which formerlyinhabited the same continent. Bearing in mind that the mutual relation oforganism to organism is of the highest importance, we can see why twoareas, having nearly the same physical conditions, should often beinhabited by very different forms of life; for according to the length oftime which has elapsed since the colonists entered one of the regions, orboth; according to the nature of the communication which allowed certainforms and not others to enter, either in greater or lesser numbers;according or not as those which entered happened to come into more or lessdirect competition with each other and with the aborigines; and accordingas the immigrants were capable of varying more or less rapidly, there wouldensue in the to or more regions, independently of their physicalconditions, infinitely diversified conditions of life; there would be analmost endless amount of organic action and reaction, and we should findsome groups of beings greatly, and some only slightly modified; somedeveloped in great force, some existing in scanty numbers--and this we dofind in the several great geographical provinces of the world.

On these same principles we can understand, as I have endeavoured to show,why oceanic islands should have few inhabitants, but that of these, a largeproportion should be endemic or peculiar; and why, in relation to the meansof migration, one group of beings should have all its species peculiar, andanother group, even within the same class, should have all its species thesame with those in an adjoining quarter of the world. We can see why wholegroups of organisms, as batrachians and terrestrial mammals, should beabsent from oceanic islands, whilst the most isolated islands shouldpossess their own peculiar species of aerial mammals or bats. We can seewhy, in islands, there should be some relation between the presence ofmammals, in a more or less modified condition, and the depth of the seabetween such islands and the mainland. We can clearly see why all theinhabitants of an archipelago, though specifically distinct on the severalislets, should be closely related to each other, and should likewise berelated, but less closely, to those of the nearest continent, or othersource whence immigrants might have been derived. We can see why, if thereexist very closely allied or representative species in two areas, howeverdistant from each other, some identical species will almost always there befound.

As the late Edward Forbes often insisted, there is a striking parallelismin the laws of life throughout time and space; the laws governing thesuccession of forms in past times being nearly the same with thosegoverning at the present time the differences in different areas. We seethis in many facts. The endurance of each species and group of species iscontinuous in time; for the apparent exceptions to the rule are so few thatthey may fairly be attributed to our not having as yet discovered in anintermediate deposit certain forms which are absent in it, but which occurabove and below: so in space, it certainly is the general rule that thearea inhabited by a single species, or by a group of species, iscontinuous, and the exceptions, which are not rare, may, as I haveattempted to show, be accounted for by former migrations under differentcircumstances, or through occasional means of transport, or by the specieshaving become extinct in the intermediate tracts. Both in time and spacespecies and groups of species have their points of maximum development. Groups of species, living during the same period of time, or living withinthe same area, are often characterised by trifling features in common, asof sculpture or colour. In looking to the long succession of past ages, asin looking to distant provinces throughout the world, we find that speciesin certain classes differ little from each other, whilst those in anotherclass, or only in a different section of the same order, differ greatlyfrom each other. In both time and space the lowly organised members ofeach class generally change less than the highly organised; but there arein both cases marked exceptions to the rule. According to our theory,these several relations throughout time and space are intelligible; forwhether we look to the allied forms of life which have changed duringsuccessive ages, or to those which have changed after having migrated intodistant quarters, in both cases they are connected by the same bond ofordinary generation; in both cases the laws of variation have been thesame, and modifications have been accumulated by the same means of naturalselection.