We are told, moreover, that, with hardly an exception, the organic forms found in a fossil state[{212}] must be supposed to be the last of their special line of development, which terminated in them; so that neither can they be claimed as the direct ancestors of any other forms, fossil or living, nor can any others which are actually known be claimed as their progenitors. The genealogies supplied for almost all known species, extinct or existing, are admittedly conjectural, and as in the most famous instance of all, namely the supposed common ancestor of simians and men, the links are persistently "missing." Thus M. de Quatrefages, speaking of the human pedigree as set forth by Professor Haeckel, writes thus:[247]
All species, existing or extinct, are said to have been preceded by ancestral forms which have disappeared without leaving the slightest vestige behind them. The amphioxus itself, which more than any other realizes the type of the group it represents, was preceded, according to Haeckel, by the provertebrate, which no man has ever seen, but of which, nevertheless, the Jena professor gives us a figure, and describes the anatomy.
Thus the number of forms postulated by the theory of genetic Evolution, must have been enormous beyond conception, in comparison with those belonging to the numerically insignificant groups which formed the mere extremities of branches on the genealogical tree.[{213}]
This being premised, we must ask what Geology has to tell us on the subject, and it will be well to begin by briefly recalling the main features of the geological record.
The stratified rocks comprising the crust of the earth, in which fossil plants and animals are found embedded, have evidently been formed at successive periods, chiefly by the agency of water, each formation having begun as a sediment like the mud or ooze at the bottom of our oceans and seas. Geological investigation has proved that the chronological order of the strata thus deposited can be satisfactorily determined, and they are found to divide themselves, in respect of the organisms they contain, into three great series, lying above the Azoic (or lifeless) rocks, older than them all.
These series, beginning from the bottom, in which order we shall have to trace their history, are most conveniently named Primary, Secondary, and Tertiary, otherwise termed respectively, Palœozoic ("ancient life"), Mesozoic ("middle life"), and Kainozoic ("recent life"). Each of these again, contains various formations, or as we may call them volumes of its chronicle, each of which has its fixed place in order of sequence.
Thus, always proceeding from below upwards, in the Primary series, commencing with the Laurentian, we find successively the Huronian, Cambrian, Silurian, Devonian or Old Red Sandstone, Carboniferous, and Permian.[{214}]
In the Secondary, the lowest formation is the Triassic or New Red Sandstone, followed by the Jurassic or Oolite, and the Cretaceous or Chalk.
Finally the Tertiary has three main divisions; the Eocene, or "dawn of the recent," Miocene, or "less recent," and Pliocene, or "more recent."
Above these comes the series now in progress, variously called, Quaternary, Post-Tertiary, and Pleistocene, or "most recent."