ANIMAL.] 283 living genera gradually appear, and true snakes (Ophidia) are first met with. Among birds, all the existing orders, many families, and some living genera appear in the Miocene period. Mammalia, however, exhibit the most surprising advance. Ancestral forms of all the existing orders are found in the Eocene formation ; in the Miocene, most living families are well developed ; while in the Pliocene and post-Pliocene deposits we find the genera and species for the most part closely resembling those that still inhabit the earth. The following diagrammatic table will enable the reader better to comprehend the main facts which we have here endeavoured to set forth. It comprises only the larger or more important groups of animals, and of each of these the known range in time is indicated by a thick line. It has not been attempted to show the breaks which occur in our knowledge of the range of a group, since no one now doubts that where any type appears in two remote periods it must have been in existence during the whole TABLE SHOWING THE RANGE IN TIME OP THE MORE IMPORTANT GROUPS OF ANIMALS. ANIMAL GROUPS. PAJUEOZOIC. MKSOZOIC. CAINOZOIC. Laurentian. Cambrian. Lower Silurian. Upper Silurian. Devonian. Carboniferous. Permian. C3 i g 5
"o O Purbeck. Wealden. Cretaceous. Eocene. Miocene. Pliocene. Post-Pliocene. J 3 M { Foraminifera PKOTOZOA J C(ELENTEUATA. ... J ECHIXODKKMATA. J CRUSTACEA , ASACHXIDA Spoil ida . . Graptolitidae
Corallaria Crinoids Asteroids.. Ecliinoids . . . Cirrliipcdia Trilobita . MerostoiuaUi Aiionioura - _ Brachyura . . .
MYIUAl DUA [XSECTA MOLLUSCA - 1 VERTEBUATA < Cephalopoda Pisces Amphibia 1 Reptilia Aves Mammalia intervening period, although we may have no record of it. Neither has it been attempted to indicate the abundance or scarcity of the group in each period, this being a detail suited only to a special treatment of science of palaeontology. It must also be remembered that it is often impossible for us to determine whether the increased prevalence of fossil remains of a particular group is due to a really greater development of the animals, or only to more favourable con ditions for their preservation and discovery. On considering the successive phases of animal life pro- sented to us by the fossil remains preserved in the rocks, we cannot help perceiving that there has been on tlu; whole a steady advance in organization and an increase in variety and complexity, from the earliest geological periods to the present day. Thus the oldest known fossil belong -s to the lowest type of animal life the Protozoa. Then we have the lower forms of Molluscs, Brachiopoda and
Pteropoda followed by the Cephalopoda and Gasteropoda.
