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. 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_. 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." * * * * * It seems advisable to begin our investigation with the vegetable kingdom, as its classification being comparatively simple, the essential points of its development are easily followed. We cannot do better than start with the summary of its main divisions furnished by Mr. Carruthers.[248]