78 PAST HISTORY OF ANIMALS. 



descendants. That a species should disappear need not 

 surprise us, if we believe in the " transformation " of one 

 species into another. The disappearance is more apparent 

 than real, the species lives on in its modified descendants, 

 " different species " though they be. 



But, on the other hand, there are not a few fossil animals 

 which have become wholly extinct, having apparently left no 

 direct descendants. Such are the ancient Trilobites 

 (perhaps remotely connected with our king crab), their 

 allies the Eurypterids, two classes of Echinoderms (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 Cuttlefish 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. 



Illusti-ations of the Appearance of Animals in Time. 



Such tallies as those given here are apt to l>e 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. Biit the general result 

 is clear. We find that in the Cambrian rocks, before I-lsfies appeared, 

 the great Invertebrate classes were represented, though as yet but 

 feebly. As we pass upwards they increase in number and' in differentia- 

 tion. 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. 



