98 The Outline of Science 



these there intervened the quiescent, well-protected pupa stage or 

 chrysalis, probably adapted to begin with as a means of surviving 

 the severe winter. For it is easier for an animal to survive when 

 the vital processes are more or less in abeyance. 



Disappearance of many Ancient Types 



We cannot leave the last period of the Palaeozoic era and its 

 prolonged ice age without noticing that it meant the entire cessa- 

 tion 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-scor- 

 pions 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 com- 

 prised in the Palaeozoic 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 



