CHAPTER XlII 

 THE DUAL RELATIONSHIPS OF ANIMALS 



A CRITICAL Study of fossil animals taken as a whole 

 brings out two apparently contradictory 

 facts. 



In the first place all the major groups of animals 

 have maintained the same relationship to each other 

 from the very first. The characteristic features of these 

 major groups have undergone no change whatever. 

 Crustaceans have always been crustaceans, echino- 

 derms have always been echinoderms, and mollusks 

 have always been mollusks. There is not the slight- 

 est evidence which supports any other viewpoint. 



Yet on the other hand within each major group 

 there has been constant and continual change from 

 age to age. All of the crustaceans, echinoderms and 

 mollusks of the present day are more or less, and often 

 very widely, different from the representatives of 

 those groups which flourished in the distant past. 



How can such a dual relationship of animal forms — 

 fixed and inflexible major groups each including con- 

 stantly changing types — be possible? Not only is it 

 possible, but it is to be inferred from the facts of 

 geology and geography as we understand and inter- 

 pret them. 



At the very earliest time at which life in any form 

 could be presumed to have existed on the earth there 

 was water in abundance, and there must have been 



