ANIMAL.] DISTRIBUTION 285 Secondary monkey, or (we may add) a Silurian bird or mammal. The following table is abbreviated from that in the Students Elements (p. 315), as it is well calculated to show how scanty and accidental is our knowledge, and how necessarily imperfect must be the geological record in still earlier periods. Number and Distribution of Fossil Mammalia from Strata older than the Tertiary, SECONDARY STRATA. Number of species. Locality. First Discovery. Maestricht chalk
White chalk _ ....
o Upper Greensand Gault
...
Neocomian (Lower )
Green sand) )
Wealden
Upper Purbeck Oolite Middle Purbeck Oolite ...
25 Swanage 1854 Lower Purbeck Oolite.... Portland Oolite.
o Kimnierid "e clay. . o Coral rag o Oxford clav o Great Oolite 4 Stonesfield 1818 Inferior Oolite . .. o Lias o Upper Trias (Somerset, ) If Carolina) 4 Wiirtemberg. 1847 Middle Trias o Lower Trias o PRIMARY STRATA. Permian o Carboniferous o Devonian o Silurian o Cambrian o Laurentian o For an account of the characteristics of these small animals, and for some details of their history, we refer the reader to Sir Charles Lyell s work ; it is here only necessary to state the circumstances under which these re mains have been preserved and discovered. Fossil remains of land animals are, of course, rarely found except in lacustrine or estuarine deposits ; and these are often en tirely wanting throughout extensive geological formations. But even where such fossiliferous beds occur, the condi tions favourable to the preservation of small Mammalia are exceedingly rare, the entire series of fresh-water Wealden beds having yielded no trace of them, although we are quite certain that they were then both varied and abundant. Even more remarkable is the fact that the whole 25 species of Purbeck mammals, belonging to 10 genera, were obtained from a single stratum only a few inches thick, and from an area of less than 500 square yards. Yet these small animals must have abounded at this period ; and it is impossible to believe that anything but a most imperfect and fractional representation of the mam malian fauna of the country could have been gathered into this narrow graveyard. But this thin stratum occurs amid a mass of fresh-water deposits 160 feet thick, the whole of which have been thoroughly and systematically examined by the officers of the Geological Survey of Great Britain ; and though many of the layers contain remains of land organisms plants, insects, and land-shells no other part of the whole series has yielded a single fragment of mammalian remains ! Having this striking example of the worthlessness of negative evidence, it behoves us to be cautious of rejecting any legitimate conclusions from the facts in our possession, on account of the absence of the direct evidence of fossil remains. The varied and highly- developed Mammalia of the Eocene period really necessitate (to the evolutionist) the long-continued previous existence of this class of animals; and the discovery of isolated species in the Oolite and Trias would (had it been delayed to our time) have been but a confirmation of theoretical deductions. In his anniversary address to the Geological Society in 1870, Professor Huxley adduces a number of special cases showing that, on the theory of development, almost all the higher forms of life must have existed during the Palaeozoic period. Thus, from the fact that almost the whole of the Tertiary period has been required to convert the ancestral Orohippus into the existing horse, he believes that, in order to have time for the much greater change of the ancestral Ungulata into the two great divisions of Perissodactyles and Artiodactyles (of which change there is no trace even among the earliest Eocene Mammals), we should require a large portion, if not the whole, of the Mesozoic period. Another case is furnished by the bats and Cetacea, which occur fully developed in the Eocene formation ; and these would have required still more time for their modification out of ancestral Insectivora and Carnivora. The Marsupials of the Trias, again, were already differentiated into herbivorous and carnivorous forms ; so that on the lowest estimate w r e must place the common ancestor of the Mammalia very far back in Palaeozoic times. Reptiles furnish evidence of the same character. Professor Huxley says, " If the very small differences which are observable between the Crocodilia of the older Mesozoic formations and those of the present day furnish any sort of approxima tion towards an estimate of the average rate of change among the Sauropsida, it is almost appalling to reflect how far back in Palaeozoic times we must go before we can hope to arrive at that common stock from which the Crocodilia, Lacertilia, Ornithoscelida, and Plesiosauria, which had attained so great a development in the Triassic epoch., must have been derived." And if to these indications we add the appearance of two orders of fishes Elasmobranchs and Ganoids in the Silurian period, we shall be compelled to place the origin of the whole vertebrate stock at an epoch far beyond that of the lowest fossiliferous rocks of the Cambrian series. If, then, we bear in mind the very early appearance of so many highly complex organisms, representing all the great types of animal life almost all the great invertebrate groups in the Cambrian and Lower Silurian, with many Vertebrata and almost all forms of Insecta in the Devonian and Car boniferous periods, while a large number of these have hardly increased in complexity of organization down to our times, we shall be prepared to admit the extreme probability of Mr Darwin s view, that "before the lowest Cambrian stra tum was deposited, long periods elapsed, as long as, or pro bably far longer, than the whole interval from the Cambrian age to the present day ; and that during these vast periods the world swarmed with living creatures " (Origin of Species, 6th ed. p. 286.) Professor Ramsay has recently expressed analogous views, founded on an extensive survey of the whole series of geological formations. In a paper " On the com parative value of certain Geological Ages (or Groups of Formations) considered as items of Geological Time" (Proceedings of the Royal Society, 1874, p. 334), he says speaking of the abundant and well-developed fauna of the Cambrian period, a sketch of which we have given at p. 282: "In this earliest known -varied life we find no evidence of its having lived near the beginning of the zoological series. In a broad sense, compared with what must have gone before both biologically and physically, all the phenomena connected with this old period seem, to my mind, to be of quite a recent description; and the climates of seas and lands were of the very same kind as
those the world enjoys at the present day."
