The Cainozoic, or Tertiary, is the age of mammals and of man. In it the great reptilian tyrants of the Mesozoic disappear, and are replaced on land and sea by mammals or beasts of the same orders with those now living, though differing as to genera and species (see Fig. 5). So greatly, indeed, did mammalian life abound in this period that in the middle part of the Tertiary most of the leading groups were represented by more numerous species than at present; while many groups then existing have now no representatives. At the close of this great and wonderful procession of living beings comes man himself--the last and crowning triumph of creation; the head, thus far, of life on the earth.
I have merely glanced at the leading events of this wonderful history, because its details may be found in so many manuals and popular works on geology. But if we imagine this great chain of life extending over periods of enormous duration in comparison with the short span of human history, presenting to the naturalist hosts of strange forms which he could scarcely have imagined in his dreams, we may understand how exciting have been these discoveries crowded within the lives of two generations of geologists. Further, when we consider that the general course of this great development of life, beginning with Protozoa and ending with man, is from below upward--from the more simple to the more complex--and that there is of necessity, in this grand growth of life through the ages, a likeness or parallelism to the growth of the individual animal from its more simple to its more complex state, we can understand how naturalists should fancy that here they have been introduced to the workshop of Nature, and that they can discover how one creature may have been developed from another by spontaneous evolution.
Skeleton of the American Mastodon, illustrating the number and wide distribution of elephantine animals of the three genera _Dinotherium_, _Mastodon_, and _Elephas_ in the later Tertiary Age. Gaudry, the most eminent modern authority on these animals, remarks that the facts at present known do not "permit us to indicate any relation of descent between the elephantine animals and those of other orders known to us at present."]