Chapter 20 Chapter 13 Geographical Distribution (Continued)

DISTRIBUTION OF FRESH WATER Organisms--Observation of Organisms on Sea Islands--Absence of Amphibians and Terrestrial Mammals--Relationship of Insular Life to that of the Nearest Continent--Transportation of Organisms from Their Nearest Origin and Their Subsequent Changes —Summary of previous and present chapters. freshwater organisms Since lakes and river systems are separated by land barriers, it might be expected that freshwater organisms would not spread widely in the same area, and since the sea is a more difficult obstacle to overcome, it would probably be expected that freshwater organisms would not expand as far as distant areas.But the opposite is true.Not only are the many freshwater species belonging to different classes widely distributed, but allied species are distributed throughout the world in an alarming manner.When I first collected various freshwater creatures in Brazil, I remember very well my amazement at how similar the freshwater insects, shellfish, etc. were to those in Britain, while the surrounding terrestrial life was not.

But with regard to the wide-spread ability of freshwater organisms, I think that in most cases it can be explained that they have become adapted in a highly Rivers frequently make short-distance migrations from one river to another; the development of this ability to wide-ranging distribution will be an almost inevitable consequence.We can consider only a few examples here; the least explainable of these are fish.It was previously believed that the same freshwater species could never exist on two continents that were far apart from each other.But Dr Ginter recently demonstrated that the milkfish (Galaxias attenuatus) inhabits Tasmania, New Zealand, the Falkland Islands and mainland South America.This is a singular instance, and may represent, during a former warm period, the distribution of this fish from the center of the Antarctic.But since species of this genus are also capable of crossing great distances by some unknown means, the Ginte case is in some ways not uncommon; About 230 miles, but both places have a common species.On the same continent fresh-water fishes are often widely distributed and very variable; for in two adjacent river systems some species are the same, and others are quite different.

Freshwater fish were transported out by accident, presumably by so-called accidental methods.For example, it is not very rare for fish to be carried up by a cyclone and land at a remote point alive; and we know that the eggs retain their vigor for a considerable time after they have been taken from the water.Nevertheless, their distribution is largely due to changes in the level of the land in recent times that have allowed rivers to communicate with each other.Also, the flow of rivers into each other occurred during the Flood, where there was no change in the level of the land.Most contiguous mountain ranges must have from ancient times completely prevented the confluence of the rivers on either side, and the great difference in the fish on either side led to the same conclusion.Some fresh-water fishes are of very ancient forms, and in such cases there has been ample time for great geographical changes, and consequently ample time and means for great migrations.Furthermore, Dr. Günther has recently inferred, from several investigations, that fishes are capable of maintaining for a long time the same form.If careful treatment is given to saltwater fishes, they will gradually become accustomed to freshwater life; and, according to Valenciennes, there is hardly any species of fish whose members live only in freshwater, and which therefore belong to the Sea-dwelling species of freshwater groups can swim very far along the coast, and probably have little difficulty becoming re-adapted to distant freshwaters.

Certain species of fresh-water mollusks are so widespread, and allied species spread throughout the world, that allied species, descended from a common ancestor, must, according to our theory, have descended from a single source.Their distribution puzzled me at first, since their eggs do not appear to be transportable by birds; and the eggs, like the adults, are immediately killed by sea water.I cannot even comprehend how some naturalized species can spread so rapidly over the same country.But two facts which I have observed—and no doubt others will be discovered—give some light on the matter.Twice I have seen duckweeds cling to their backs when ducks popped out of ponds covered in duckweed; I have inadvertently moved shellfish from one aquarium to another while in another aquarium.But there is another medium that may be more effective: I hang a duck's foot in an aquarium in which many freshwater shellfish eggs are hatching; I find many extremely small, newly hatched shellfish crawling on its feet, and are so firmly attached thereto, that they do not fall off when the feet are out of the water, though they will fall of themselves when they are a little older.These newly hatched molluscs, though water-dwelling in their nature, live on their feet in moist air for twelve to twenty hours; Or herons can fly six or seven hundred miles at least; and if they are blown across the sea to an oceanic island or other distant point, they will necessarily land in a pond or creek.Sir Lyall told me that he had once caught a Dytiscus, to which was firmly attached the shield snail (Ancylus, a fresh-water shell like a limpet); (Colymbetes), once flew aboard the Beagle, which was at that time forty-five miles from the nearest land: no one can say how far it may have been carried with a fair wind.

With regard to plants, it has long been known that many freshwater, and even swampy, species are widely distributed, both on continents and on the most remote oceanic islands.According to De Candolle, this is markedly exhibited by large groups of terrestrial plants with few aquatic members; for they seem to acquire at once a large range by virtue of their aquatic habitat.This fact, I think, can be accounted for by the method of distribution which is favorable.I've said before that small bits of dirt sometimes cling to birds' feet and beaks.Waders often hang around the muddy edges of ponds, and if they suddenly startle and fly, they probably have mud on their feet.Birds of this order roam more widely than any other; they sometimes come to the most remote and barren islands of the sea; they probably do not alight on the surface of the sea, so that any soil from their feet is not washed off; Once on land, they inevitably fly to their natural freshwater habitat.I do not believe botanists appreciate how many seeds are contained in the mud of a pond; I have made several small experiments, but I can only give one of the most touching: I grew from a small Three teaspoons of sludge were taken from three different places underwater at the edge of the pond, which weighed only six and three-quarters ounces when dry; I covered it and kept it in my study for six months, when each plant grew When it comes out, pull it out and count it; the plants are of many species, 537 in all; and that slimy sludge would fit in a breakfast mug!In view of these facts, it seems to me inexplicable if water-birds do not transport the seeds of fresh-water plants to unvegetated ponds and rivers in remote places.The same medium would presumably work on the eggs of some small freshwater animals.

Other unknown agents presumably also played a role.I have said that freshwater fish eat certain kinds of seeds, although they swallow many others and spit them out; even small fish swallow quite large seeds, such as yellow water lilies and eyeweeds Seeds of the genus Potamogeton.Herons and other birds have eaten fish every day for centuries and centuries; The seeds excreted in feces still retain the ability to germinate.When I formerly saw the large seeds of the delicate lotus (Nelumbium), and remembered Candor's opinion on the distribution of this plant, I thought its method of distribution must be incomprehensible; but Audubon says, He has found the seeds of the southern lotus (probably, according to Dr. Hooker, the large North American yellow lotus [Neiumbium luteum]) in the stomach of a heron.This bird must often fly to a distant pond after filling its stomach with food, and then feast on fish, and analogy leads me to believe that it deposits seeds suitable for germination in clumps of dung. discharge.

When considering these several methods of distribution, it should be remembered that a pond or a river, for example, on a raised island, is initially formed without living organisms in it; a single seed or egg will then obtain Good chance of success.There is always a struggle for life among the living beings in the same pond, however few the species may be, but the number of species even in a pond full of living beings is always small compared with the number of species inhabiting the same area of ​​land, so , the competition among them is less severe than that between terrestrial species; consequently alien aquatic invaders have a better chance of acquiring new positions than land settlers.We should also remember that many freshwater productions are lower in natural systems, and we have reason to believe that such are more slowly modified than higher ones; this allows time for the migration of aquatic species.We should not forget that many fresh-water forms have probably formerly been distributed continuously over large areas, and then became extinct at intermediate points.But the fresh-water plants and lower animals, whether they remain of the same type or vary in some degree, evidently depend chiefly on animals in their distribution, especially on those species which are strong in flight, and which fly naturally from one body of water to another. freshwater birds disperse their seeds and eggs widely.

On the Creatures of the Sea Islands Not only have all the individuals of the same species migrated from a single country, but all allied species now inhabiting the most distant points have all migrated from a single country, the birthplace of their early ancestors. I now discuss the last of the three classes of facts that I have selected to be the most difficult about distributions.I have given my reasons for not believing that, during the existence of species, the continents have been extended on such a great scale, that all the islands in these oceans were thereby filled with the present terrestrial productions.This view removes many difficulties, but is inconsistent with all the facts concerning island life.In what follows, I shall not confine myself to the question of distribution, but shall also discuss certain other cases in connection with the truth of the doctrine of independent creation, and of descent with modification.

The species of all classes inhabiting sea-islands are scarce in number compared with those of similarly sized continental areas: this fact is admitted by De Candol on plants, and Wollaston on insects.New Zealand, for instance, with its high mountains and varied terrain, and stretching 780 miles from north to south, together with the outlying islands of Auckland, Campbell, and Chatham, has altogether but 960 species of phenotypic plants; if we Comparing this modest number with species inhabiting an equivalent area of ​​South-West Australia, or the Cape of Good Hope, we must admit that some cause, independent of different physical conditions, has given rise to such a great difference in the number of species.Even Cambridge, in the same conditions, has 847 species of plants, and the Isles of Anglesey 764, but a number of ferns and introduced plants are included in these numbers, and the comparison is otherwise not very apt.We have evidence to say that the barren island of Ascension originally contained less than half a dozen species of flowering plants; but many species have now become naturalized there, as do many plants in New Zealand, and in every other sea that can be named. The same is true for island naturalization.At St. Helena, there is reason to believe that naturalized plants and animals have nearly wiped out or totally wiped out many native organisms.Whoever accepts the doctrine that each species was created separately must admit that a sufficiently large number of the most adapted plants and animals were not created for sea islands; More fully and more perfect than nature does.

Although the number of species on oceanic islands is low, the proportion of endemic species (that is, species found nowhere else in the world) is often disproportionately large.For example, if we compare the number of endemic terrestrial molluscs on Madeira, or endemic birds of the Galapagos Islands, with those found on any continent, and then compare the size of these islands with the Compare the areas and we will see that this is true.This fact is theoretically to be expected, for, as has already been shown, the occasional arrival of a species after a long interval into a new isolated region is bound to compete with new inhabitants, and is very liable to variation, And will often produce groups of mutant offspring.It must not, however, be held that the species of one class on an island are almost peculiar, and that all the species of other classes, or of other parts of the same class, must also be peculiar; Having been imported en masse, their mutual relations with one another have not been much disturbed; partly by the frequent importation of unchanged species from their native places, with which the productions of the island have crossed.It should be remembered that the vigor of the offspring of such a cross must be increased; so that even an accidental cross will have a greater effect than expected.I will give some examples to illustrate the above point: There are 26 species of land-birds in the Galapagos Islands; of these 21 (or 23) are peculiar, and of the 11 sea-birds only two are peculiar. and it is evident that sea-birds reach these islands more easily and more often than terrestrial birds.On the other hand, the distance between Bermuda and North America is almost the same as the distance between the Galapagos Islands and South America, and Bermuda has a very special soil, but it does not have a unique terrier We know from Mr. J. M. Jones' admirable report on Bermuda that many birds of North America visit this island occasionally, or even regularly.According to Mr. EV Harcourt (Mr. EV Harcourt), almost every year, many European and African birds are blown to Madeira by the wind; this island is inhabited by 99 kinds of birds, only one of which is special, Although it is closely related to a European type; three or four other species are found only on this island and the Canary Islands.The islands of Bermuda and Madeira, therefore, are filled with birds from the neighboring continents, which for a long time have struggled there, and become adapted to each other.After settling, therefore, in a new home, each species will be maintained in its proper place and habits by the others, and the result will not be liable to change.Any propensity to vary is also checked by crossing with unmodified importants, often from the original country.Furthermore, Madeira is inhabited by a surprising number of special terrestrial molluscs, but none of the marine molluscs is unique to the ocean here: now, although we do not know how the marine molluscs are distributed, we can know Their eggs or larvae, attached to seaweed or floating wood, or to the feet of waders, can be transported across three or four hundred miles of ocean, much more easily than terrestrial shellfish.The different orders of insects that inhabit Madeira show an almost parallel situation.

Oceanic islands sometimes lack whole classes of animals, and their places are taken by other classes; Location.Although New Zealand is here discussed as an island of the sea, it is somewhat doubtful whether it should be so; its size is large, and there is no deep sea separating it from Australia; The character of the island and the orientation of the mountains, Reverend Clark recently argued, should be regarded as dependencies of Australia, along with New Caledonia.As regards plants, Dr. Hooker has shown that the proportions of the different orders in the Galapagos Islands are very different from their proportions elsewhere.All these quantitative differences, and the absence of whole groups of certain animals and plants, are generally explained by supposed differences in the physical conditions of the islands; but this explanation is very doubtful.Ease of emigration seems to be as important as the nature of the conditions. There are also many little things to notice about the creatures of Sea Island.For example, on certain islands where no mammal inhabits, some endemic plants have wonderful hooked seeds; but the purpose of the hooks is to carry the seeds away by the hair or fur of a quadruped, and nothing less This relationship is more obvious.But the hooked seed could presumably be brought to an island by other means; and the plant would then be modified and endemic, retaining its hook, which would become a useless appendage. Animals, like many island insects, still have shriveled wings beneath their healed elytra.Moreover, islands are often inhabited by trees or shrubs, to which orders elsewhere include only herbaceous species; and trees, as de Candolle has shown, generally have a limited range, whatever the cause.Trees are therefore seldom likely to reach remote oceanic islands; herbaceous plants have no chance of competing successfully with the many fully developed trees that grow on the continent, and the herbaceous plants, once settled Tall and dominant over other herbaceous plants.In this case, whatever order the plant belongs to, natural selection will tend to increase its height, thus making it first a shrub and then a tree. Amphibians and terrestrial mammals are absent from oceanic islands Regarding the absence of whole orders on oceanic islands, Saint Vincent said long ago that the oceans are dotted with many islands, but amphibians (frogs, toads, salamanders) have never been found.I have painstakingly attempted to confirm this statement, and have found that, apart from New Zealand, New Caledonia, the Andaman Islands, or perhaps also the Salomon and Seychelles islands, This statement is correct.But I once said that whether New Zealand and New Caledonia should be listed as ocean islands is still doubtful; as for whether Andaman, Solomon Islands and Seychelles should be listed as ocean islands, it is even more doubtful.The general absence of frogs, toads, and salamanders on so many true oceanic islands cannot be explained by the physical conditions of oceanic islands; indeed, islands seem especially adapted to such animals: for frogs have been brought into Madeira, to the Azores and Mauritius, where they multiply to the point of being a nuisance.But because these animals and their eggs die immediately when they encounter sea water (as far as we know, there is an exception for one Indian species), they are difficult to transport across the sea, so we can know why they do not exist in real oceans island.But why they were not created there is difficult to explain on the basis of creationism. Mammals provide another similar situation.I have searched carefully for the oldest nautical records, and have not found a single unmistakable instance of terrestrial mammals (other than domestic animals kept by natives) inhabiting islands more than 300 miles from the mainland or large land islands. ; on many of the islands closer to the mainland as well.The Falkland Islands have a wolf-like fox, very much like an exception; but this group of islands cannot be regarded as an island of the sea, because it lies on a sandbar connected to the mainland at a distance of about 280 miles; have carried boulders to its west coast, and they may have carried foxes there before too, as is often the case in the Arctic.It cannot be said, however, that small islands cannot support at least small mammals, which in many parts of the world live on small islands close to the mainland; and hardly a single island can be named where our small quadrupeds cannot be found. morphed and multiplied greatly.On the general view of creationism, it cannot be said that there was not enough time for the creation of mammals; many volcanic islands are very old, as can be seen from the great erosion they have suffered, and from their Tertiary formations: there There was still time enough for the production of endemic species belonging to other classes; and we know that new species of mammals are produced and eliminated on continents at a faster rate than other animals inferior to them.While terrestrial mammals are not found on oceanic islands, aerial mammals are found on almost every island.New Zealand has two species of bats that are not found anywhere else in the world: Norfolk Island, Viti, Bonin, Caroline and Marianne, Mauritius, all There are their specialty bats.It may be asked: Why did that supposed creativity produce bats and not other mammals on a remote island?In my opinion the question is easy to answer; for no terrestrial animal can cross the wide spaces of the sea, but bats can.Bats have been seen flying far across the Atlantic during the day; and two North American bats have either regularly or occasionally traveled as far as Bermuda, 600 miles from the mainland.I have heard from Mr. Tomes, who specializes in this family of animals, that many of the species of this family have a wide range, and can be found on the continent and on remote islands.We need only suppose, therefore, that such wandering species vary in their new home due to their new position, and we can thus understand why oceanic islands, with their native endemic bats, are devoid of all other bats. Terrestrial mammals. There is also an interesting relationship between the depth of water that separates islands from each other or from the nearest continent and the degree of their mammalian kinship.Mr. Windsor Earl has made some fascinating observations on this subject, which have since been greatly extended by the admirable studies of Mr. Wallace in the great Malay Archipelago, which is represented by a deep-sea space. Adjacent to Celebes, this deep sea separates two very different worlds of mammals.The sea is rather shallow on either side of these islands, which are inhabited by identical or closely allied quadrupeds.I have not yet had time to study the situation of this problem in all parts of the world; but as far as I have studied, the relationship is correct.For example, Britain and Europe are separated by a shallow sea, and the mammals are the same on both sides; and so are all the islands near the coast of Australia.On the other hand, the West Indies lie on very deep sand-bars, to a depth of almost 1,000 fathoms, where we find the American forms, but very different species and even genera.As the amount of change in all species of animals depends partly on the length of time, and as islands isolated by shallow seas or from the mainland are more likely to be united in modern times than islands isolated by deep seas, we can understand that in the What relation exists between the depth of sea which separates two groups of mammalian fauna and the degree of their relatedness--a relation which, on theories of independent creation, is quite implausible. The above is a statement of the life of the islands of the sea—that is, the species are few in number, and the endemic forms predominate locally—the membership of some groups varies, while the members of other groups of the same class do not vary—certain orders, Such as amphibians and terrestrial mammals, all absent, although flying bats are present - some plant orders show special proportions - herbaceous types develop into trees, etc. - There are two explanations for these problems The belief, that the means of occasional transport in the long course of time are valid, and that all sea-islands were formerly united with the nearest continent, seems to me to be more true than the latter.For, on the latter view, presumably the different classes will have imported more uniformly, and since the species have imported collectively, their mutual relations will not be greatly disturbed, so that either they do not change, or all the species compare with each other. Changes happen in the same way.I do not deny that there are many serious difficulties in the understanding of the comprehension of the comparison of how many of the organisms of distant islands (either remaining in the same species of form or later changing) have ever reached their present homes.It must not be overlooked, however, that other islands which once served as resting places may now leave little trace, and I would like to detail a difficult example.Nearly all sea islands, even the most isolated and smallest, are inhabited by terrestrial shellfish, generally endemic species, but sometimes found elsewhere—in this respect Dr. Gould cites Give a moving example from the Pacific Ocean.Terrestrial molluscs are known to be easily killed by sea water; their eggs, at least those I have tested, sink in sea water and are killed.But there must be some unknown and occasionally efficient way of transporting them.Are the newly hatched larvae sometimes attached to the feet of birds perched on the ground and thus transported there?I am reminded of terrestrial molluscs with membranes over the mouths of their shells during dormancy, which can float across fairly wide bays in crevices of driftwood.And I have found several species which have been submerged in this state for seven days without injury: a Roman snail (Helix pomatia), thus treated, and placed in dormancy for another twenty days in sea water, can completely resurrection.During such a long period, the shellfish may be carried by currents of average speed to a distance of about 660 geographical miles.As this Roman snail has a thick calcareous operculum, I removed the operculum, and after a new membrane had formed, I immersed it in sea water for fourteen days, and it revived and crawled away.Baron Aucaptaine later conducted a similar experiment: he put 100 land shells belonging to ten species in boxes covered with many small holes, and put the boxes in the sea for fourteen days.Of the one hundred shellfish, twenty-seven came to life.The presence of the shell seems to be important, since of the twelve Cyclostoma elegans with the shell, eleven survived.It is worth noting that the Roman snail species I tested was very good at resisting sea water, while none of the other four Roman snail species tested by Ogabeden could be revived out of fifty-four specimens.However, transport of terrestrial shellfish is by no means entirely dependent on this method; the feet of birds offer a more likely method. The relationship of island organisms to those on the nearest continent The most moving and important fact to us is that the species inhabiting islands are related to species which are not actually identical to those of the nearest continent.Innumerable examples could be given of this.The Galapagos Islands lie below the equator, 500 to 600 miles off the coast of South America.Almost every land and water creature there bears the unmistakable imprint of the American continent.There are twenty-six species of land-birds; of these twenty-one or twenty-three are ranked as distinct species, and are generally supposed to have been created there; Close kinship is shown in every character, as in their habits, posture, and song.The same is true of the other animals, and of the greater part of the plants in Dr. Hooker's admirable flora of the archipelago.When the naturalist observes life on these volcanic islands of the Pacific, hundreds of miles from the mainland, he feels himself standing on the American continent.Why is this the case?Why suppose that the species created in the Galapagos Islands, and not elsewhere, are so clearly related to those created in America?Nothing in the conditions of life, in the geological character of the island, in its height or climate, or in the proportion of the several classes which inhabit it, closely resembles those on the South American coast: in fact , there are considerable differences in all these respects.The Galapagos and Cape Verde Islands, on the other hand, have a considerable resemblance in the volcanic nature of the soil, in the climate, in the altitude, and in the size of the islands: but how completely and absolutely Different!The life of the Cape Verde Islands is related to that of Africa, as the life of the Galapagos is related to that of America.Facts of this kind cannot be explained on the general view of independent creation; , whether this is due to accidental delivery methods or due to previously continuous landmass (although I don't believe in this theory).And the Cape Verde Islands receive settlers from Africa; though such settlers are liable to vary—yet the principle of heredity betrays the place of their origin. Many similar facts could be cited: it is indeed an almost universal law that the peculiar life of an island is associated with that of the nearest continent, or of the nearest large island.Exceptions are few, and most of them are explainable.Thus, although the Keghlan land is closer to Africa than to America, we know from Dr. Hooker's report that its plants are closely related to those of America: This exception can be explained from the point of view that the seeds were brought along with the soil and stones by icebergs carried by periodic ocean currents.New Zealand is more closely related endemically to the nearest continent, Australia, than it is to any other region: this is perhaps to be expected, but it is also clearly related to South America, which although is the second nearest continent, but is so far away that this fact is an exception.But this difficulty partly disappears from the view that some parts of the life of New Zealand, South America, and other southern lands have descended from a nearly intermediate, though remote, point, the Antarctic Islands, which are Before the warmer Tertiary and the beginning of the last ice age, the Antarctic islands were overgrown with vegetation.The affinities of the flora of the South-West Point of Australia and the Cape of Good Hope, though weak, are the more noteworthy when Dr. Hooker has convinced me that the affinities are real; No doubt it will be explained in the future. The same laws which determine the affinities between the organisms of islands and those of the nearest continent, may sometimes manifest themselves, in small but interesting ways, within the limits of the same archipelago.It is a curious fact, for example, that on each of the separate islands of the Galapagos Archipelago inhabit each separate island many different species; but these species are more related to one another than they are to the life of the Organisms in other regions are more closely related.This was probably to be expected, since islands so near to each other would almost necessarily receive settlers from the same source, and from each other.But how were the different, though not very different, variations among the many settlers on islands facing each other, having the same geological character, height, climate, etc.?This has long been a difficulty for me: but it arises chiefly from the deep-rooted erroneous notion that the physical conditions of a region are of most importance; it cannot be disproved, however, that each species must compete with other species, and therefore The properties of other species are at least as important, and generally more important factors of success.Now, if we look at the species that inhabit the Galapagos Islands and are also found in other parts of the world, we can see that they vary considerably on several islands.If island life had been brought by accidental means of transport—say, the seeds of one plant had been brought to one island, and the seeds of another plant to another, though all seeds are all derived from the same source; then the above-mentioned differences are indeed to be expected.因此，一种移住者在以前时期内最初在诸岛中的一个岛上定居下来时，或者它以后从一个岛散布到另一个岛上时，它无疑会遭遇到不同岛上的不同条件，因为它势必要与一批不同的生物进行竞争；比方说，一种植物在不同的岛上会遇到最适于它的土地已被多少不同的物种所占据，并且还会受到多少不同的敌人的打击。如果在那个时候这物种变异了，自然选择大概就会在不同岛上引起不同变种的产生。尽管如此，有些物种还会散布开去并且在整个群中保持同一的性状，正如我们看到在一个大陆上广泛散布的物种保持着同一性状一样。 在加拉帕戈斯群岛的这种情形里以及在程度较差的某些类似的情形里，真正奇异的事实是，每一个新物种在任何一个岛上一旦形成以后，并不迅速地散布到其他岛上。但是，这些岛，虽然彼此相望，却被很深的海湾分开，在大多数情形里比不列颠海峡还要宽，并且没有理由去设想它们在任何以前的时期是连续地连结在一起的。在诸岛之间海流是迅速的和急激的，大风异常稀少；所以诸岛彼此的分离远比地图上所表现的更加明显。虽然如此，有些物种以及在世界其他部分可以找到的和只见于这群岛的一些物种，是若干岛屿所共有的；我们根据它们现在分布的状态可以推想，它们是从一个岛上散布到其他岛上去的。但是，我想，我们往往对于密切近似物种在自由往来时，便有彼此侵占对方领土的可能性，采取了错误的观点。毫无疑问，如果一个物种比其他物种占有任何优势，它就会在很短的时间内全部地或局部地把它排挤掉；但是如果两者能同样好地适应它们的位置，那么两者大概都会保持它们各自的位置到几乎任何长的时间。经过人的媒介而归化的许多物种曾经以惊人的速度在广大地区里进行散布，熟悉了这种事实，我们就会容易推想大多数物种也是这样散布的；但是我们应该记住，在新地区归化的物种与本地生物一般并不是密切近似的，而是很不相同的类型，如得康多尔所阐明的，在大多数情形下是属于不同的属的。在加拉帕戈斯群岛，甚至许多鸟类，虽然那么适于从一个岛飞到另一个岛，但在不同的岛上还是不相同的；例如，效舌鸫（mocking-thrush）有三个密切近似的物种，每一个物种只局限于自己的岛上。现在，让我们设想查塔姆岛的效舌鸫被风吹到查理士岛（Charles），而后者已有另一种效舌鸫：为什么它应该成功地定居在那里呢、我们可以稳妥地推论，查理士岛已经繁生着自己的物种，因为每年有比能够养育的更多的蛋产生下来和更多的幼鸟孵化出来；并且我们还可以推论，查理士岛所特有的效舌鸫对于自己家乡的良好适应有如查塔姆岛所特有的物种一样。莱尔爵士和沃拉斯顿先生曾经写信告诉我一个与本问题有关的可注意的事实；即马德拉和附近的圣港（Porto Santo）小岛具有许多不同的而表现为代表物种的陆栖贝类，其中有些是生活在石缝里的；虽然有大量为石块每年从圣港输送到马德拉，可是马德拉并没有圣港的物种移住进来；虽然如此，两方面的岛上都有欧洲的陆栖贝类栖息着，这些贝类无疑比本地物种占有某些优势。根据这些考察，我想，我们对于加拉帕戈斯群岛的若干岛上所特有的物种并没有从一个岛上散布到其他岛上的事，就不必大惊小怪了。再者，在同一大陆上，“先行占据”对于阻止在相同物理条件下栖息的不同地区的物种混入，大概有重要的作用。例如，澳洲的东南部和西南部具有几乎相同的物理条件，并且由一片连续的陆地联络着，可是它们有巨大数量的不同哺乳类，不同鸟类和植物栖息着；据贝茨先生说，栖息在巨大的、开阔的、连续的亚马逊谷地的蝴蝶和其他动物的情形也是这样。 支配海洋岛生物的一般特性的这同一原理，即移住者与它们最容易迁出的原产地的关系，以及它们以后的变异，在整个自然界中有着广泛的应用。我们在每一山顶上、每一个湖泊和沼泽里都可看到这个原理，因为高山物种，除非同一物种在冰期已经广泛散布，都与周围低地的物种是相关联的；这样，南美洲的高山蜂鸟（humming-birds）、高山啮齿类、高山植物等，一切都严格地属于美洲的类型；而且显然地，当一座山缓慢隆起时，生物便会从周围的低地移来。湖泊和沼泽的生物也是这样，除非极方便的输送允许同一类型散布到世界的大部分。从美洲和欧洲洞穴里的大多数盲目动物的性状也可看到这同一原理。还能举出其他类似的事实。我相信，以下情形将被认为是普遍正确的，即在任何两个地区，不问彼此距离多少远，凡有许多密切近似的或代表的物种存在，在那里便一定也有某些相同的物种；并且不管在什么地方，凡有许多密切近似的物种，在那里也必定有被某些博物学者列为不同物种而被其他博物学者仅仅列为变种的许多类型；这些可疑的类型向我们示明了变异过程中的步骤。 某些物种在现在或以前时期中的迁徒能力和迁徙范围，与密切近似物种在世界遥远地点的存在有一定的关系，这种关系还可用另一种更加普通的方式表示出来。古尔德先生很久以前告诉我，在世界各处散布的那些鸟属中，许多物种分布范围是广阔的。我不能怀疑这条规律是普遍正确的，虽然它很难被证明。在哺乳类中，我们看见这条规律显著地表现在蝙蝠中，并以较小的程度表现在猫科和狗科里。同样的规律也表现在蝴蝶和甲虫的分布上。淡水生物的大多数，也是这样，因为在最不同的纲里有许多属分布在世界各处，而且它们的许多物种具有广大的分布范围。这并不是说在分布很广的属里一切物种都有很广阔的分布范围，而是说其中某些物种有很广阔的分布范围。这也不是说在这样的属里物种平均有很广阔的分布范围；因为这大部分要看变化过程进行的程度；比方说，同一物种的两个变种栖息在美洲和欧洲，因此这个物种就有很广的分布范围；但是，如果变异进行得更远一些，那两个变种就会被列为不同的物种，因而它们的分布范围就大大地缩小了。这更不是说能越过障碍物而分布广远的物种，如某些善飞的鸟类，就必然分布得很广，因为我们永远不要忘记，分布广远不仅意味着具有越过障碍物的能力，而且意味着具有在遥远地区与异地同住者进行生存斗争并获得胜利的这种更加重要的能力。但是按照以下的观点——一属的一切物种，虽然分布到世界最遥远的地点，都是从单一祖先传下来的；我们就应该找到，并且我相信我们确能照例找到，至少某些物种是分布得很广远的。 我们应该记住，在一切纲里许多属的起源都是很古的，在这种情形下，物种将有大量的时间可供散布和此后的变异。从地质的证据看来，也有理由相信，在每一个大的纲里比较低等的生物的变化速率，比起比较高等的生物的变化速率更加缓慢；结果前者就会分布广远而仍然保持同一物种性状的较好机会。这个事实以及大多数低级体制类型的种籽和卵都很细小并且较适于远地输送的事实，大概说明了一个法则，即任何群的生物愈低级，分布得愈广远；这是一个早经观察到的、并且最近又经得康多尔在植物方面讨论过的法则。 刚刚讨论过的关系——即低等生物比高等生物的分布更加广远——分布广远的属，它的某些物种的分布也是广远的，——高山的、湖泊的和沼泽的生物一般与栖息在周围低地和干地的生物有关联，——岛上和最近大陆上的生物之间有显著关系，——在同一群岛中诸岛上的不同生物有更加密切的亲缘关系，——根据各个物种独立创造的普通观点，这些事实都是得不到解释的，但是如果我们承认从最近的或最便利的原产地的移居以及移居者以后对于它们的新家乡的适应，这就可以得到解释。 前章和本章提要 在这两章里我曾竭力阐明，如果我们适当地估计到我们对于在近代必然发生过的气候变化和陆地水平变化以及可能发生过的其他变化所产生的充分影响是无知的，——如果我们记得我们对于许多奇妙的偶然输送方法是何等无知——如果我们记得，而且这是很重要的一点，一个物种在广大面积上连续地分布，而后在中间地带绝灭了，是何等常常发生的事情，——那么，相信同一物种的一切个体，不管它们是在哪里发现的，都传自共同的祖先，就没有不可克服的困难了。我们根据各种一般的论点，特别是根据各种障碍物的重要性，并且根据亚属、属和科的相类似的分布，得出上述结论，许多博物学者在单一创造中心的名称下也得出这一结论。 至于同一属的不同物种，按照我们的学说，都是从一个原产地散布出去的；如果我们像上述那样地估计到我们的无知，并且记得某些生物类型变化得很缓慢，因而有大量时间可供它们迁徙，那么难点决不是不能克服的；虽然在这种情形下，就像在同一物种的个体的情形下一样，难点往往是很大的。 为了说明气候变化对于分布的影响，我曾经试图阐明最后的一次冰期曾经发生过多么重要的作用，它甚至影响到赤道地区，并且它在北方和南方寒冷交替的过程中让相对两半球的生物互相混合，而且把一些生物留在世界的所有部分的山顶上。为了说明偶然的输送方法是何等各式各样，我曾经略为详细地讨论了淡水生物的散布方法。 如果承认同一物种的一切个体以及同一属的若干物种在时间的悠久过程中曾经从同一原产地出发，并没有不可克服的难点；那么一切地理分布的主要事实，都可以依据迁徙的理论，以及此后新类型的变异和繁生，得到解释。这样，我们便能理解，障碍物，不问水陆，不仅在分开而且在显然形成若干动物区域和植物区域上，是有高度重要作用的。这样，我们还能理解同一地区内近似动物的集中化，比方说在南美洲，平原和山上的生物，森林、沼泽和沙漠的生物，如何以奇妙的方式彼此相关联，并且同样地与过去栖息在同一大陆上的绝灭生物相关联。如果记住生物与生物之间的相互关系是最高度重要的，我们就能知道为什么具有几乎相同的物理条件的两个地区常常栖息着很不相同的生物类型；因为根据移住者进入一个或两个地区以来所经过的时间长度；根据交通性质所容许的某些类型而不是其他类型以或多或少的数量迁入；根据那些移入的生物是否波此以及与本地生物进行或多或少的直接竞争：并且根据移人的生物发生变异的快慢，所以在两个地区或更多的地区里就会发生与它们的物理条件无关的无限多样性的生活条件，——根据这种种情况，那里就会有一个几乎无限量的有机的作用和反作用，——并且我们就会发见某些群的生物大大地变异了，某些群的生物只是轻微地变异了，——某些群的生物大量发展了，某些群的生物仅以微小的数量存在着，——我们的确可以在世界上几个大的地理区里看到这种情形。 依据这些同样的原理，如我曾经竭力阐明的，我们便能理解，为什么海洋岛只有少数生物，而这些生物中有一大部分又是本地所特有的，即特殊的；由于与迁徙方法的关系，为什么一群生物的一切物种都是特殊的，而另一群生物、甚至同纲生物的一切物种都与邻近地区的物种相同。我们能够知道，为什么整个群的生物，如两栖类和陆栖哺乳类，不存在于海洋岛上。同时最孤立的岛也有它们自己特有的空中哺乳类即蝙蝠的物种。我们还能够知道，为什么在岛上存在的或多或少经过变异的哺乳类和这些岛与大陆之间的海洋深度有某种关系。我们能够清楚地知道，为什么一个群岛的一切生物，虽然在若干小岛上具有不同的物种，然而彼此有密切的关系；并且和最近大陆或移住者发源的其他原产地的生物同样地有关系，不过关系较不密切。我们更能知道，两个地区内，不论相距多么远，如果有很密切近似的或代表的物种存在，为什么在那里总可以找到相同的物种。 正如已故的福布斯所经常主张的，生命法则在时间和空间中有一种显著的平行现象；支配生物类型在过去时期内演替的法则与支配生物类型在今日不同地区内的差异的法则，几乎是相同的。在许多事实中我们可以看到这种情形。在时间上每一物种和每一群物种的存在都是连续的；因为对这一规律的显然例外是这么少，以致这些例外可以正当地归因于我们还没有在某一中间的沉积物里发现某些类型，这些类型不见于这种沉积物之中，却见于它的上部和下部：在空间，也是这样的，即，一般规律肯定是，一个物种或一群物种所栖息的地区是连续的，而例外的情形虽然不少，如我曾经企图阐明的，都可以根据以前在不同情况下的迁徒、或者根据偶然的输送方法、或者根据物种在中间地带的绝灭而得到解释。在时间和在空间里，物种以及物种群都有它们发展的最高点。生存在同一时期中的或者生存在同一地区中的物种群，常常有共同的微细特征，如刻纹或颜色。当我们观察过去悠久的连续时代时，正如观察整个世界的遥远地区，我们发现某些纲的物种彼此之间的差异很小，而另一纲的、或者只是同一日的不同组的物种彼此之间的差异却很大。在时间和在空间里，每一纲的低级体制的成员比高级体制的成员一般变化较少；但是在这两种情形里，对于这条规律都有显著的例外。按照我们的学说，在时间和在空间里的这些关系是可以理解的；因为不论我们观察在连续时代中发生变化的近缘生物类型或者观察迁入遥远地方以后曾经发生变化的近缘生物类型，在这两种情形里，它们都被普通世代的同一个纽带连结起来；在这两种情形里，变异法则都是一样的，而且变异都是由同一个自然选择的方法累积起来的。