Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/165

Rh Cretaceous period we leave the Secondary Age, and pass on to the Tertiary.

The Tertiary Age is subdivided into the Eocene, Miocene, and Pliocene periods. These names were chosen to express the result of a comparison made between the shells found in the rocks of the Tertiary formation and those living at the present day, the object in view being to determine whether many living shells are found petrified in the Tertiary rocks. Thus, in Sicily, of one hundred petrified shells, from seventy to ninety are found in existing seas; hence the name Pliocene, or most recent, was given to rocks containing such a large proportion of living shells. Those parts of the Tertiary formation known as Miocene, or less recent, have from forty to fifty per cent., while only the dawn of recent shells is expressed by the term Eocene. The subdivision of the Tertiary Age into these three periods, originally based on the proportion between the fossil and living shells, was afterward applied to Tertiary plants and animals generally, it being supposed that a proportion similar to that of fossil and living shells existed between Tertiary plants and animals and those of the present day. These periods often, however, pass so gradually into one another, the lines of demarkation not being very well defined, that this classification is not always applicable. The Tertiary Age, notwithstanding the minor differences of its periods, is essentially an age of Mammals, Palms, and Exogens. There is no necessity of describing the details of its animal and vegetal life, since Asia and Africa, with their Hippopotami, Rhinoceroses, Elephants, Lions, and Tigers, living amidst the characteristic tropical plants give one an excellent idea of what America, Great Britain, etc. were during their Tertiary Ages. To the Evolutionist the