The Outline of Science, Volume 1

We cannot leave the last period of the Pal ozoic era and its prolonged ice age without noticing that it meant the entire cessation of a large number of ancient types, especially among plants and backboneless animals, which now disappear for ever. It is necessary to understand that the animals of ancient days stand in three different relations to those of to-day. (a) There are ancient types that have living representatives, sometimes few and sometimes many, sometimes much changed and sometimes but slightly changed. The lamp-shell, Lingulella, of the Cambrian and Ordovician period has a very near relative in the Lingula of to-day. There are a few extremely conservative animals. (b) There are ancient types which have no living representatives, except in the guise of transformed descendants, as the King-crab (Limulus) may be said to be a transformed descendant of the otherwise quite extinct race to which Eurypterids or Sea-scorpions belonged. (c) There are altogether extinct types—lost races—which have left not a wrack behind. For there is not any representation to-day of such races as Graptolites and Trilobites. Looking backwards over the many millions of years comprised in the Pal ozoic era, what may we emphasise as the most salient features? There was in the Cambrian the establishment of the chief classes of backboneless animals; in the Ordovician the first fishes and perhaps the first terrestrial plants; in the Silurian the emergence of air-breathing Invertebrates and mud-fishes; in the Devonian the appearance of the first Amphibians, from which all higher land animals are descended, and the establishment of a land flora; in the Carboniferous the great Club-moss forests[Pg 99] and an exuberance of air-breathing insects and their allies; in the Permian the first reptiles and a new flora.