84 LIFE ON THE EARTH. 



For example, Cephalopoda, by universal consent, 

 stand at the head of the Molluscous kingdom of 

 animals; but their origin is of the same date as 

 that of the Mollusca generally : they rapidly rise to 

 importance, but pass the maximum in the Mesozoic 

 period, and are now but a small and scattered part of 

 the inhabitants of the sea, enormously outnumbered 

 by the inferior races of Gasteropoda and Dimyaria. 

 Thus, starting from an equal basis, the superior class 

 has lost in the 'struggle for existence.' But we 

 must examine this subject on other occasions, after 

 gathering additional data. 



CHANGES IN MARINE ANIMALS WITH ELAPSED 

 TIME. 



The principal classes of marine fossil Invertebrata 

 have now been traced from what seems to be their 

 origin, to or beyond the epoch of their greatest 

 prevalence. We have, in fact, taken the census of 

 our marine inhabitants at several periods. It re- 

 mains to examine them with reference to their 

 structure and grade of organization in these periods ; 

 to compare, for instance, the Crustacea and Mollusca 

 of one period with those of another, and thus to 

 learn the amount of variation in this respect from 

 period to period, and what is the method of varia- 

 tion. We may include in this inquiry Fishes, which 



