Mr. Darwin says that "in the Southern hemisphere, if we compare large tracts of land in Australia, South Africa, and western South America, between latitudes 25° and 35°, we shall find parts extremely similar in all their conditions, yet it would not be possible to point out three faunas and floras more utterly dissimilar. Or again we may compare the productions of South America south of lat. 35° with those north of 25°, which consequently inhabit a considerably different climate, and they will be found incomparably more closely related to each other, than they are to the productions of Australia or Africa under nearly the same climate." Still more striking are the contrasts which Mr. Darwin points out between adjacent areas that are totally cut off from each other. "No two marine faunas are more distinct, with hardly a fish, shell, or crab in common, than those of the eastern and western shores of South and Central America; yet these great faunas are separated only by the narrow, but impassable, isthmus of Panama." On opposite sides of high mountain-chains, also, there are marked differences in the organic forms—differences not so marked as where the barriers are absolutely impassable, but much more marked than are necessitated by unlikenesses of physical conditions.
Not less suggestive is the converse fact that wide geographical areas which offer decided geologic and meteorologic contrasts, are peopled by nearly-allied groups of organisms, if there are no barriers to migration. "The naturalist in travelling, for instance, from north to south never fails to be struck by the manner in which successive groups of beings, specifically distinct, yet clearly related, replace each other. He hears from closely allied, yet distinct kinds of birds, notes nearly similar, and sees their nests similarly constructed, but not quite alike, with eggs coloured in nearly the same manner. The plains near the Straits of Magellan are inhabited by one species of Rhea (American Ostrich), and northward the plains of La Plata by another species of the same genus; and not by a true ostrich or emu, like those found in Africa and Australia under the same latitude. On these same plains of La Plata, we see the agouti and bizcacha, animals having nearly the same habits as our hares and rabbits and belonging to the same order of Rodents, but they plainly display an American type of structure. We ascend the lofty peaks of the Cordillera and we find an alpine species of bizcacha; we look to the waters, and we do not find the beaver or musk-rat, but the coypu and capybara, rodents of the American type. Innumerable other instances could be given. If we look to the islands off the American shore, however much they may differ in geological structure, the inhabitants, though they may be all peculiar species, are essentially American."
What is the generalization implied by these two groups of facts? On the one hand, we have similarly-conditioned, and sometimes nearly-adjacent, areas, occupied by quite different Faunas. On the one hand, we have areas remote from one another in latitude, and contrasted in soil as well as climate, occupied by closely-allied Faunas. Clearly then, 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. The organisms do no occur in such and such places solely because they are either specially fit for those places, or more fit for them than all other organisms.
The induction under which these facts come, and which unites them with various other facts, is a totally-different one. When we see 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; we are at once reminded of the general truth exemplified in the last section—the 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. And we are shown that through these perpetually-recurring attempts to thrust itself into every accessible habitat, each species spreads until it reaches limits which are for the time insurmountable.
§ 107. We pass now to the distribution of organic forms in Time. Geological inquiry has established the truth that during a Past of immeasurable duration, plants and animals have existed on the Earth. In all countries their buried remains are found in greater or less abundance. From comparatively small areas multitudinous different types have been exhumed. Every exploration of new areas, and every closer inspection of areas already explored, brings more types to light. And beyond question, an exhaustive examination of all exposed strata, and of all strata now covered by the sea, would disclose types immensely out-numbering those at present known. Further, geologists agree that even had we before us every kind of fossil which exists, we should still have nothing like a complete index to the past inhabitants of our globe. Many sedimentary deposits have been so altered by the heat of adjacent molten matter, as greatly to obscure the organic remains contained in them. The extensive formations once called "transition," and now re-named "metamorphic," are acknowledged to be formations of sedimentary origin, from which all traces of such fossils as they probably included have been obliterated by igneous action. And the accepted conclusion is that igneous rock has everywhere resulted from the melting-up of beds of detritus originally deposited by water. How long the reactions of the Earth's molten nucleus on its cooling crust, have been thus destroying the records of Life, it is impossible to say; but there are strong reasons for believing that the records which remain bear but a small ratio to the records which have been destroyed. Thus we have but extremely imperfect data for conclusions respecting the distribution of organic forms in Time. Some few generalizations, however, may be regarded as established.
One is that the plants and animals now existing mostly differ from the plants and animals which have existed. Though there are species common to our present Fauna and to past Faunas, yet the facies of our present Fauna differs, more or less, from the facies of each past Fauna. On carrying out the comparison, we find that past Faunas differ from one another, and that the differences between them are proportionate to their degrees of remoteness from one another in Time, as measured by their relative positions in the sedimentary series. So that if we take the assemblage of organic forms living now, and compare it with the successive assemblages of organic forms which have lived in successive geologic epochs, we find that the farther we go back into the past, the greater does the unlikeness become. The number of species and genera common to the compared assemblages, becomes smaller and smaller; and the assemblages differ more and more in their general characters. Though a species of brachiopod now extant is almost identical with a species found in Silurian strata, though between the Silurian Fauna and our own there are sundry common genera of molluscs, yet it is undeniable that there is a proportion between lapse of time and divergence of organic forms.
This divergence is comparatively slow and continuous where there is continuity in the geological formations, but is sudden, and comparatively wide, wherever there occurs a great break in the succession of strata. The contrasts which thus arise, gradually or all at once, in formations that are continuous or discontinuous, are of two kinds. Faunas of different eras are distinguished partly by the absence from the one of type's present in the other, and partly by the unlikenesses between the types common to both. Such contrasts between Faunas as are due to the appearance or disappearance of types, are of secondary significance: they possibly, or probably, do not imply anything more than migrations or extinctions. The most significant contrasts are those between successive groups of organisms of the same type. And among such, as above said, the differences are, speaking generally, small and continuous where a series of conformable strata gives proof of continued existence of the type in the locality; while they are comparatively large and abrupt where the adjacent formations are shown to have been separated by long intervals.
Another general fact, referred to by Mr. Darwin as one which palæontology has made tolerably certain, is that forms and groups of forms which have once disappeared from the Earth, do not reappear. Passing over the few species which have continued throughout the whole period geologically recorded, it may be said that each species after arising, spreading for an era, and continuing abundant for an era, eventually declines and becomes extinct; and that similarly, each genus during a longer period increases in the number of its species, and during a longer period dwindles and at last dies out. After making its exit neither species nor genus ever re-enters. The like is true even of those larger groups called orders. Four types of reptiles which were once abundant have not been found in modern formations, and do not at present exist. Though nothing less than an exhaustive examination of all strata, can prove conclusively that a type of organization when once lost is never reproduced, yet so many facts point to this inference that its truth can scarcely be doubted.
To frame a conception of the total amount and general direction of the change in organic forms during the time measured by our sedimentary series, is at present impossible—the data are insufficient. The immense contrast between the few and low forms of the earliest-known Fauna, and the many and high forms of our existing Fauna, has been commonly supposed to prove, not only great change but great progress. Nevertheless, this appearance of progress may be, and probably is, mainly illusive. Wider knowledge has shown that remains of comparatively well-organized creatures really existed in strata long supposed to be devoid of them, and that where they are absent, the nature of the strata often explains their absence, without assuming that they did not exist when these strata were formed. It is a tenable hypothesis that the successively-higher types fossilized in our successively-later deposits, indicate nothing more than successive migrations from pre-existing continents to continents that were step by step emerging from the ocean—migrations which necessarily began with the inferior orders of organisms, and included the successively-superior orders as the new lands became more accessible to them and better fitted for them.[[43]]
While the evidence usually supposed to prove progression is thus untrustworthy, there is trustworthy evidence that there has been, in many cases, little or no progression. Though the orders which have existed from palæozoic and mesozoic times down to the present day, are almost universally changed, yet a comparison of ancient and modern members of these orders shows that the total amount of change is not relatively great, and that it is not manifestly towards a higher organization. Though nearly all the living forms which have prototypes in early formations differ from these prototypes specially, and in most cases generically, yet ordinal peculiarities are, in numerous cases, maintained from the earliest times geologically recorded, down to our own time; and we have no visible evidence of superiority in the existing genera of these orders. In his lecture "On the Persistent Types of Animal Life," Prof. Huxley enumerated many cases. On the authority of Dr. Hooker he stated "that there are Carboniferous plants which appear to be generically identical with some now living: that the cone of the Oolitic Araucaria is hardly distinguishable from that of an existing species; that a true Pinus appears in the Purbecks and a Juglans in the chalk." Among animals he named palæozoic and mesozoic corals which are very like certain extant corals; genera of Silurian molluscs that answer to existing genera; insects and arachnids in the coal-formations that are not more than generically distinct from some of our own insects and arachnids. He instanced "the Devonian and Carboniferous Pleuracanthus, which differs no more from existing sharks than these do from one another;" early mesozoic reptiles "identical in the essential characters of their organization with those now living;" and Triassic mammals which did not differ "nearly so much from some of those which now live, as these differ from one another." Continuing the argument in his "Anniversary Address to the Geological Society" in 1862, Prof. Huxley gave many cases in which the changes that have taken place, are not changes towards a more specialized or higher organization—asking "in what sense are the Liassic Chelonia inferior to those which now exist? How are the Cretaceous Ichthyosauria, Plesiosauria, or Pterosauria less embryonic or more differentiated species than those of the Lias?" While, however, contending that in most instances "positive evidence fails to demonstrate any sort of progressive modification towards a less embryonic or less generalized type in a great many groups of animals of long-continued geological existence," Prof. Huxley added that there are other groups, "co-existing with them under the same conditions, in which more or less distinct indications of such a process seem to be traceable." And in illustration of this, he named that better development of the vertebræ which characterizes some of the more modern fishes and reptiles, when compared with ancient fishes and reptiles of the same orders; and the "regularity and evenness of the dentition of the Anoplotherium as contrasting with that of existing Artiodactyles."[[44]]