§ 136. The general truths of morphology thus coincide in their implications. Unity of type, maintained under extreme dissimilarities of form and mode of life, is explicable as resulting from descent with modification; but is otherwise inexplicable. The likenesses disguised by unlikenesses, which the comparative anatomist discovers between various organs in the same organism, are worse than meaningless if it be supposed that organisms were severally framed as we now see them; but they fit in quite harmoniously with the belief that each kind of organism is a product of accumulated modifications upon modifications. And the presence, in all kinds of animals and plants, of functionally-useless parts corresponding to parts that are functionally-useful in allied animals and plants, while it is totally incongruous with the belief in a construction of each organism by miraculous interposition, is just what we are led to expect by the belief that organisms have arisen by progression.
CHAPTER VII.
THE ARGUMENTS FROM DISTRIBUTION.
§ 137. In [§§ 105] and [106], we contemplated the phenomena of distribution in Space. The general conclusions reached, in great part based on the evidence brought together by Mr. Darwin, were that, "on the one hand, we have similarly-conditioned, and sometimes nearly-adjacent, areas, occupied by quite different Faunas. On the other hand, we have areas remote from each other in latitude, and contrasted in soil as well as climate, which are occupied by closely-allied Faunas." Whence it was inferred that "as like organisms are not universally, or even generally, found in like habitats; nor very unlike organisms, in very unlike habitats; there is no manifest pre-determined adaptation of the organisms to the habitats." In other words, the facts of distribution in Space do not conform to the hypothesis of design. At the same time we saw that "the similar areas peopled by dissimilar forms, are those between which there are impassable barriers; while the dissimilar areas peopled by similar forms, are those between which there are no such barriers;" and these generalizations appeared to harmonize with the abundantly-illustrated truth, "that each species of organism tends ever to expand its sphere of existence—to intrude on other areas, other modes of life, other media."
By way of showing still more clearly the effects of competition among races of organisms, let me here add some recently-published instances of the usurpations of areas, and changes of distribution hence resulting. In the Natural History Review for January, 1864, Dr. Hooker quotes as follows from some New Zealand naturalists:—"You would be surprised at the rapid spread of European and other foreign plants in this country. All along the sides of the main lines of road through the plains, a Polygonum (aviculare), called 'Cow Grass,' grows most luxuriantly, the roots sometimes two feet in depth, and the plants spreading over an area from four to five feet in diameter. The dock (Rumex obtusifolius or R. crispus) is to be found in every river bed, extending into the valleys of the mountain rivers, until these become mere torrents. The sow-thistle is spread all over the country, growing luxuriantly nearly up to 6000 feet. The water-cress increases in our still rivers to such an extent, as to threaten to choke them altogether ... I have measured stems twelve feet long and three-quarters of an inch in diameter. In some of the mountain districts, where the soil is loose, the white clover is completely displacing the native grasses, forming a close sward.... In fact, the young native vegetation appears to shrink from competition with these more vigorous intruders." "The native (Maori) saying is 'as the white man's rat has driven away the native rat, so the European fly drives away our own, and the clover kills our fern, so will the Maoris disappear before the white man himself.'"
Given this universal tendency of the superior to overrun the habitats of the inferior,[[50]] let us consider what, on the hypothesis of evolution, will be the effects on the geographical relationships of species.
§ 138. A race of organisms cannot expand its sphere of existence without subjecting itself to new external conditions. Those of its members which spread over adjacent areas, inevitably come in contact with circumstances partially different from their previous circumstances; and such of them as adopt the habits of other organisms, necessarily experience re-actions more or less contrasted with the re-actions before experienced. Now if changes of organic structure are caused, directly or indirectly, by changes in the incidence of forces; there must result unlikenesses of structure between the divisions of a race which colonizes new habitats. Hence, in the absence of obstacles to migration, we may anticipate manifest kinships between the animals and plants of one area, and those of areas adjoining it. This inference corresponds with an induction before set down ([§ 106]). In addition to illustrations of it already quoted from Mr. Darwin, his pages furnish others. One is that species which inhabit islands are allied to species which inhabit neighbouring main lands; and another is that the faunas of clustered islands show marked similarities. "Thus the several islands of the Galapagos Archipelago are tenanted," says Mr. Darwin, "in a quite marvellous manner, by very closely related species; so that the inhabitants of each separate island, though mostly distinct, are related in an incomparably closer degree to each other than to the inhabitants of any other part of the world." Mr. Wallace has traced "variation as specially influenced by locality" among the Papilionidæ inhabiting the East Indian Archipelago: showing how "the species and varieties of Celebes possess a striking character in the form of the anterior wings, different from that of the allied species and varieties of all the surrounding islands;" and how "tailed species in India and the western islands lose their tails as they spread eastward through the archipelago." During his travels on the Upper Amazons, Mr. Bates found that "the greater part of the species of Ithomiæ changed from one locality to another, not further removed than 100 to 200 miles;" that "many of these local species have the appearance of being geographical varieties;" and that in some species "most of the local varieties are connected with their parent form by individuals exhibiting all the shades of variation."
Further general relationships are to be inferred. If races of organisms, ever being thrust by pressure of population into new habitats, undergo modifications of structure as they diverge more and more widely in Space, it follows that, speaking generally, the widest divergences in Space will indicate the longest periods during which the descendants from a common stock have been subject to modifying conditions; and hence that, among organisms of the same group, the smaller contrasts of structure will be limited to the smaller areas. This we find: "varieties being," as Dr. Hooker says in his Flora of Tasmania, "more restricted in locality than species, and these again than genera." Again, if races of organisms spread, and as they spread are altered by changing incident forces; it follows that where the incident forces vary greatly within given areas, the alterations will be more numerous than in equal areas which are less-variously conditioned. This, too, proves to be the fact. Dr. Hooker points out that the relatively uniform regions have the fewest species; while in the most multiform regions the species are the most numerous.
§ 139. Let us consider next, how the hypothesis of evolution corresponds with the facts of distribution, not over different areas but through different media. If all forms of organisms have descended from some primordial form, it follows that since this primordial form must have inhabited some one medium out of the several media now inhabited, the peopling of other media by its descendants implies migration from one medium to others—implies adaptations to media quite unlike the original medium. To speak specifically—water being the medium in which the lowest living forms exist, the implication is that the earth and the air have been colonized from the water. Great difficulties appear to stand in the way of this assumption. Ridiculing those who alleged the uniserial development of organic forms, who, indeed, laid themselves open to ridicule by their many untenable propositions, Von Baer writes—"A fish, swimming towards the shore desires to take a walk, but finds his fins useless. They diminish in breadth for want of use, and at the same time elongate. This goes on with children and grandchildren for a few millions of years, and at last who can be astonished that the fins become feet? It is still more natural that the fish in the meadow, finding no water, should gape after air, thereby, in a like period of time developing lungs; the only difficulty being that in the meanwhile, a few generations must manage without breathing at all." Though, as thus presented, the belief in a transition looks laughable; and though such derivation of terrestrial vertebrates by direct modification of piscine vertebrates, is untenable; yet we must not conclude that no migrations of the kind alleged can have taken place. The adage that "truth is stranger than fiction," applies quite as much to Nature in general as to human life. Besides the fact that certain fish actually do "take a walk" without any obvious reason; and besides the fact that sundry kinds of fish ramble about on land when prompted by the drying-up of the waters they inhabit; there is the still more astounding fact that one kind of fish climbs trees. Few things seem more manifestly impossible, than that a water-breathing creature without efficient limbs, should ascend eight or ten feet up the trunk of a palm; and yet the Anabas scandens does as much. To previous testimonies on this point Capt. Mitchell has recently added others. Such remarkable cases of temporary changes of media, will prepare us for conceiving how, under special conditions, permanent changes of media may have taken place; and for considering how the doctrine of evolution is elucidated by them.
Inhabitants of the sea, of rivers, and of lakes, are many of them left from time to time partially or completely without water; and those which show the power to change their media temporarily or permanently, are in very many cases of the kinds most liable to be thus deserted by their medium. Let us consider what the sea-shore shows us. Twice a day the rise and the fall of the tide covers and uncovers plants and animals, fixed and moving; and through the alternation of spring and neap tides, it results that the exposure of the organisms living low down on the beach, varies both in frequency and duration: while some of them are left dry only once a fortnight for a very short time, others, a little higher up, are left dry during two or three hours at several ebb tides every fortnight. Then by small gradations we come to such as, living at the top of the beach, are bathed by salt-water only at long intervals; and still higher to some which are but occasionally splashed in stormy weather. What, now, do we find among the organisms thus subject to various regular and irregular alterations of media? Besides many plants and many fixed animals, we find moving animals of numerous kinds; some of which are confined to the lower zones of this littoral region, but others of which wander over the whole of it. Omitting the humbler types, it will suffice to observe that each of the two great sub-kingdoms, Mollusca and Arthropoda, supplies examples of creatures having a wide excursiveness within this region. We have gasteropods which, when the tide is down, habitually creep snail-like over sand and sea-weed, even up as far as high-water mark. We have several kinds of crustaceans, of which the crab is the most conspicuous, running about on the wet beach, and sometimes rambling beyond the reach of the water. And then note the striking fact that each of the forms thus habituated to changes of media, is allied to forms which are mainly or wholly terrestrial. On the West Coast of Ireland marine gasteropods are found on the rocks three hundred feet above the sea, where they are only at long intervals wetted by the spray; and though between gasteropods of this class and land-gasteropods the differences are considerable, yet the land-gasteropods are more closely allied to them than to any other Mollusca. Similarly, the two highest orders of crustaceans have their species which live occasionally, or almost entirely, out of the water: there is a kind of lobster in the Mauritius which climbs trees; and there is the land-crab of the West Indies, which deserts the sea when it reaches maturity and re-visits it only to spawn. Seeing, thus, how there are many kinds of marine creatures whose habitats expose them to frequent changes of media; how some of the higher kinds so circumstanced, show a considerable adaptation to both media; and how these amphibious kinds are allied to kinds that are mainly or wholly terrestrial; we shall see that the migrations from one medium to another, which evolution pre-supposes, are by no means impracticable. With such evidence before us, the assumption that the distribution of the Vertebrata through media so different as air and water, may have been gradually effected in some analogous manner, would not be altogether unwarranted even had we no clue to the process. We shall find, however, a tolerably distinct clue. Though rivers, and lakes, and pools, have no sensible tidal variations, they have their rises and falls, regular and irregular, moderate and extreme. Especially in tropical climates, we see them annually full for a certain number of months, and then dwindling away and drying up. The drying up may reach various degrees and last for various periods. It may go to the extent only of producing a liquid mud, or it may reduce the mud to a hardened, fissured solid. It may last for a few days or for months. That is to say, aquatic forms which are in one place annually subject to a slight want of water for a short time, are elsewhere subject to greater wants for longer times: we have gradations of transition, analogous to those which the tides furnish. Now it is well known that creatures inhabiting such waters have, in various degrees, powers of meeting these contingencies. The contained fish either bury themselves in the mud when the dry season comes, or ramble in search of other waters. This is proved by evidence from India, Guiana, Siam, Ceylon; and some of these fish, as the Anabas scandens, are known to survive for days out of the water. But the facts of greatest significance are furnished by an allied class of Vertebrata, almost peculiar to habitats of this kind. The Amphibia are not, like fish, usually found in waters that are never partially or wholly dried up; but they nearly all inhabit waters which, at certain seasons, evaporate, in great measure or completely—waters in which most kinds of fish cannot exist. And what are the leading structural traits of these Amphibia? They have two respiratory systems—pulmonic and branchial—variously developed in different orders; and they have two or four limbs, also variously developed. Further, the class Amphibia consists of two groups, in one of which this duality of the respiratory system is permanent, and the development of the limbs always incomplete; and in the other of which the branchiæ disappear as the lungs and limbs become fully developed. The lowest group, the Perennibranchiata, have internal organs for aerating the blood which approach in various degrees to lungs, until "in the Siren, the pulmonic respiration is more extensive and important than the branchial;" and to these creatures, having a habitat partially aërial and partially aquatic, there are at the same time supplied, in the shallow water covering soft mud, the mechanical conditions which render swimming difficult and rudimentary limbs useful. In the higher group, the Caducibranchiata, we find still more suggestive transformations. Having at first a structure resembling that which is permanent in the perennibranchiate amphibian, the larva of the caducibranchiate amphibian pursues for a time a similar life; but, eventually, while the branchial appendages dwindle the lungs grow: the respiration of air, originally supplementary to the respiration of water, predominates over it more and more, till it replaces it entirely; and an additional pair of legs is produced. This having been done, the creature either becomes, like the Triton, one which quits the water only occasionally; or, like the Frog, one which pursues a life mainly terrestrial, and returns to the water now and then. Finally, if we ask under what conditions this metamorphosis of a water-breather into an air-breather completes itself, the answer is—it completes itself at the time when the shallow pools inhabited by the larvæ are being dried up, or in danger of being dried up, by the summer's sun.[[51]]