EXTINCTION OF TYPES. 77. 



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 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. We 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 Am- 

 phibians, Amphibians are historically older than Reptiles, and many 

 types of Reptiles are much older than Birds. In short, in the couise 

 of the ages life has been slowly creeping upwards. 



[Tables. 



