EXTINCTION OF TYPES. Si 



no direct descendants. Such are the Graptolites, the 

 ancient Trilobites, their allies the Eurypterids, two classes 

 of Echinodernis (Cystoids and Blastoids), many giant 

 Reptiles, and some Mammals. 



It is almost certain that there has been no sudden 

 extinction of any animal type. There is no evidence of 

 universal cataclysm, though local floods, earthquakes, and 

 volcanic eruptions occurred in the past, as they do still, 

 with disastrous results to fauna and flora. In many cases 

 the waning away of an order, or even of a class of animals, 

 may be associated with the appearance of some formidable 

 new competitors ; thus cuttle-fish would tend to exterminate 

 Trilobites, just as man is rapidly and often inexcusably 

 annihilating many kinds of beasts and birds. Apart from 

 the struggle with competitors, it is conceivable that some 

 stereotyped animals were unable to accommodate themselves 

 to changes in their surroundings, and also that some fell 

 victims to their own constitutions, becoming too large, too 

 sluggish, too calcareous, in short, too extreme. 



Appearance of animals in time. Such tables as those given here are 

 apt to be misleading, in that they convey the impression that the 

 great types of structure have appeared suddenly. It must be noted 

 that any apparent abruptness is merely due to incompleteness of 

 knowledge or inaccuracy of expression. The table is a mere list of a 

 few important historical events, but one must fully realise that they 

 are not isolated facts, that the present lay hidden in the past and has 

 gradually grown out of it. Of the relative length of the periods 

 represented here we know almost nothing, and we are also ignorant of 

 the earliest ages in which life began. But the general result is clear. 

 AYe find that in the Cambrian rocks, before Fishes appeared, the great 

 Invertebrate classes were represented, though as yet but feebly. As 

 we pass upwards they increase in number and in differentiation. 

 Again, Fishes precede Amphibians, Amphibians are historically older 

 than Reptiles, and many types of Reptiles are much older than Birds. 

 In short, in the course of the ages life has been slowly creeping upwards. 



[TABLES. 



