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The supposition that all these types were rapidly differentiated out of Lacertilia, in the time represented by the passage from the Palaeozoic to the Mesozoic formation, appears to me to be hardly more credible, to say nothing of the indications of the existence of Dinosaurian forms in the Permian rocks which have already been obtained.
Cassian beds, for example, Palaeozoic and Mesozoic forms are commingled, and, between the Cretaceous and the Eocene formations, there are similar transitional beds. On the other hand, in the middle of the Silurian series, extensive unconformity of the strata indicates the lapse of vast intervals of time between the deposit of successive beds, without any corresponding change in the Fauna.
In the older tertiaries, the places of existing animals and plants are taken by other forms, as numerous and diversified as those which live now in the same localities, but more or less different from them; in the mesozoic rocks, these are replaced by others yet more divergent from modern types; and, in the paleozoic formations, the contrast is still more marked.
Once more, the prevalence of Macrurous before Brachyurous Podophthalmia is, apparently, a fair piece of evidence in favour of progressive modification in the same order of Crustacea; and yet the case will not stand much sifting, seeing that the Macrurous Podophthalmia depart as far in one direction from the common type of Podophthalmia, or from any embryonic condition of the Brachyura, as the Brachyura do in the other; and that the middle terms between Macrura and Brachyura the Anomura are little better represented in the older Mesozoic rocks than the Brachyura are.
The wonder is that you are ever anything else.... Do you realize that a few million generations ago, everything that stirs in us, everything that exalts human life, self-devotions, heroisms, the utmost triumphs of art, the love for love it is that makes you and me care indeed for the fate and welfare of all this round world, was latent in the body of some little lurking beast that crawled and hid among the branches of vanished and forgotten Mesozoic trees?
The late able ichthyologist Heckel pointed out the fact, that, while the Pycnodonts never possess true vertebral centra, they differ in the degree of expansion and extension of the ends of the bony arches of the vertebrae upon the sheath of the notochord; the Carboniferous forms exhibiting hardly any such expansion, while the Mesozoic genera present a greater and greater development, until, in the Tertiary forms, the expanded ends become suturally united so as to form a sort of false vertebra.
THE MESOZOIC PENEPLAIN. The immense thickness of the Mesozoic formations conveys to our minds some idea of the vast length of time involved in the slow progress of its successive ages. The same lesson is taught as plainly by the amount of denudation which the lands suffered during the era.
The Mesozoic era has three periods, the Triassic, Jurassic and Cretaceous, while the Cenozoic era names five periods, the Eocene, Oligocene, Miocene, Pliocene and Pleistocene. Absence of Certain Strata.
In the Triassic epoch, the distribution of the dry land and of terrestrial vertebrate life appears to have been, generally, similar to that which existed in the Mesozoic epoch; so that the Triassic continents and their faunae seem to be related to the Mesozoic lands and their faunae, just as those of the Miocene epoch are related to those of the present day.
I. Primary Plutonic rocks. II. Secondary Plutonic rocks. III. Tertiary Plutonic rocks. IV. Post-tertiary Plutonic rocks. 1. Primary fossiliferous or Palaeozoic strata. 2. Secondary or Mesozoic strata. 3. Tertiary or Cainozoic strata. 4. Post-tertiary strata. The oldest Plutonic rock, No. I, has been upheaved at successive periods until it has become exposed to view in a mountain-chain.
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