We have already seen that a large proportion of the living quadrupeds of Mongolia (34 out of 48) are specifically identical with those at present inhabiting the continent of Western Europe and the British Isles.
A monograph on the hippopotamus, bear, ox, stag, or any other genus of mammalia common in the European drift or caverns, might equally well illustrate the defective state of the materials at present at our command. We are rarely in possession of one perfect skeleton of any extinct species, still less of skeletons of both sexes, and of different ages. We usually know nothing of the geographical varieties of the Pleistocene and Pliocene species, least of all, those successive changes of form which they must have undergone in the preglacial epoch between the Upper Miocene and Pleistocene eras. Such being the poverty of our palaeontological data, we cannot wonder that osteologists are at variance as to whether certain remains found in caverns are of the same species as those now living; whether, for example, the Talpa fossilis is really the common mole, the Meles morreni the common badger, Lutra antiqua the otter of Europe, Sciurus priscus the squirrel, Arctomys primigenia the marmot, Myoxus fossilis the dormouse, Schmerling's Felis engihoulensis the European lynx, or whether Ursus spelaeus and Ursus priscus are not extinct races of the living brown bear (Ursus arctos).
If at some future period all the above-mentioned species should be united with their allied congeners, it cannot fail to enlarge our conception of the modifications which a species is capable of undergoing in the course of time, although the same form may appear absolutely immutable within the narrow range of our experience.
LONGEVITY OF SPECIES IN THE MAMMALIA.
In the "Principles of Geology," in 1833,* I stated that the longevity of species in the class mollusca exceeded that in the mammalia. It has been since found that this generalisation can be carried much farther, and that in fact the law which governs the changes in organic being is such that the lower their place in a graduated scale, or the simpler their structure, the more persistent are they in form and organisation. I soon became aware of the force of this rule in the class mollusca, when I first attempted to calculate the numerical proportion of Recent species in the Newer Pliocene formations as compared to the Older Pliocene, and of them again as contrasted with the Miocene; for it appeared invariably that a greater number of the lamellibranchs could be identified with living species than of the gasteropods, and of these last a greater number in the lower division, that of entire-mouthed univalves, than in that of the siphonated. In whatever manner the changes have been brought about, whether by variation and natural selection, or by any other causes, the rate of change has been greater where the grade of organisation is higher.
(* 1st edition volume 3 pages 48 and 140.)
It is only, therefore, where there is a full representation of all the principal orders of mollusca, or when we compare those of corresponding grade, that we can fully rely on the percentage test, or on the proportion of Recent to extinct species as indicating the relation of two groups to the existing fauna.
The foraminifera which exemplify the lowest stage of animal existence exhibit, as we learn from the researches of Dr. Carpenter and of Messrs. Jones and Parker, extreme variability in their specific forms, and yet these same forms are persistent throughout vast periods of time, exceeding, in that respect, even the brachiopods before mentioned.
Dr. Hooker observes, in regard to plants of complex floral structure, that they manifest their physical superiority in a greater extent of variation and in thus better securing a succession of race, an attribute which in some senses he regards as of a higher order than that indicated by mere complexity or specialisation of organ.*
(* "Introductory Essay to the Flora of Australia" page 7.)