On the problem of the extinction of species, little, demon stratively, can be said ; and on the more mysterious subject of their coming into being, no light has yet been thrown by experiment or observation. As a cause of extinction in times anterior to man, it is most reasonable to assign the chief weight to those gradual changes in the conditions affect ing a due supply of sustenance to animals in a state of nature which must have accompanied the slow alternations of land and sea brought about in the aeons of geological time. Yet this reasoning is applicable only to land-animals; for it is scarcely conceivable that such operations can have affected sea-fishes. There are characters in land-animals rendering them more obnoxious to extirpating influences, which may explain why so many of the larger species of par ticular groups have become extinct, whilst smaller species of equal antiquity have survived. In proportion to its bulk is the difficulty of the contest which, as a living organism, the individual of such species has to maintain against the sur rounding agencies that are ever tending to dissolve the vital bond, and subjugate the living matter to the ordinary chemical and physical forces. Any changes, therefore, in such external agencies as a species may have been originally adapted to exist in, will militate against that existence in a degree proportionate to the bulk of the species. If a dry season be gradually pro longed, the large Mammal will suffer from the drought sooner than the small one ; if such alteration of climate affect the quantity of vegetable food, the bulky Herbivore will first feel the effects of stinted nourishment ; if new enemies be intro duced, the large and conspicuous animal will fall a prey, while the smaller kinds conceal themselves and escape. Small quadrupeds are more prolific than large ones. Those of the bulk of the Mastodons, Megatheria, Glyptodons, and Diproto dons are uniparous. The actual presence, therefore, of small species of animals in countries where larger species of the same natural families formerly existed, is not the consequence of degeneration—of any gradual diminution of the size of such species—but is the result of circumstances which may be il lustrated by the fable of the "oak and the reed;" the smaller and feebler animals have bent and accommodated themselves to changes to which the larger species have succumbed.
That species, or forms so recognized by their distinctive characters and the power of propagating them, have ceased to exist, and have successively passed away, is a fact no longer questioned. That they have been exterminated by exceptional cataclysmal changes of the earth's surface has not been proved. That their limitation in time, in some instances or in some measure, may be due to constitutional changes accumulating by slow degrees in the long course of generations, is possible. But all hitherto observed causes of extirpation point either to continuous slowly operating geological changes, or to no greater sudden cause than the, so to speak, spectral appear ance of mankind on a limited tract of land not before inhabited. It is most probable, therefore, that the extinction of species, prior to man's presence or existence, has been due to ordinary causes—ordinary in the sense of agreement with the laws of organization and of the never-ending mutation of the geo graphical and climatal conditions on the earth's surface. The
species, and individuals of species, least adapted to bear such influences, and incapable of modifying their organization in agreement therewith, have perished. Extinction, therefore, on this hypothesis, implies the want of self-adjusting power in the individuals of the species subject thereto.
But admitting extinction as a natural law, which has operated from the beginning of life under specific forms of plants and animals, it might be expected that some evidence of it should occur in our own time, or within the historical period. Reference has been made to several instances of the extirpation of species, certainly, probably, or possibly, due to the direct agency of man. The hook-billed parrot (Nestor productus) of Philip's Island, west of New Zealand, is, perhaps, the latest instance of this kind. But this cause avails not in the question of the extinction of species at periods prior to any evidence of human existence; it does not help us in the explanation of the majority of extinctions, as of the races of aquatic Invertebrata and Vertebrata which have successively passed away.
The Great Auk (Alca impennis, L) seems to be rapidly verging to extinction. It has not been specially hunted down, like the dodo and dinornis, but by degrees has become more scarce. Some of the geological changes affecting circum stances favourable to the well-being of the A lea impennis, have been matters of observation. The last great auks, known with anything like certainty to have been seen living, were two which were taken in 1844 during a visit made to the high rock, called " Eldey," or " Meelsoekten," lying off Cape Reyki anes, the S. W. point of Iceland. This is one of three principal rocky islets formerly existing in that direction, of which the one specially named from this rare bird " Geirfugla Sker " sank to the level of the surface of the sea during a volcanic disturb ance in or about the year 1830. Such disappearance of the fit and favourable breeding-places of the Alca impennis must form an important element in its decline towards extinction. The numbers of the bones of Alea impennis on the shores of Iceland, Greenland, and Denmark, attest the abundance of the bird in former times.
Within the last century, academicians of Petersburg and good naturalists described and gave figures of the bony and the perishable parts, including the alimentary canal, of a large and peculiar fucivorous Sirenian—an amphibious animal like the Manatee, which Cuvier classified with his herbivorous Cetacea, and called Rytina Stelleri, after its discoverer. This animal inhabited the Siberian shores and the mouths of the great rivers there disemboguing. It is now believed to be extinct, and this extinction appears not to have been due to any special quest and persecution by man. We may discern in this fact the operation of changes in physical geography, which have at length so affected the conditions of existence of the Siberian manatee as to have caused its extinction. Such changes had operated, at an earlier period, to the extinction of the elephant and rhinoceros of the same region and latitudes : a future generation of zoologists may have to record the final disappear ance of the Arctic buffalo (Ovibos moschatus). Remains of Ovibos and Rytina show that they were contemporaries of Elephas primigenius and _Rhinoceros tichorrhinus.