ZOOGENESIS 



sidered belongs to it alone. From this it is clear that 

 each of these groups is safe from complete extinction 

 by any of the others. 



The relative number of different forms in the various 

 groups may vary from time to time and from place to 

 place. We see this in the present seas where the 

 mollusks are mainly creatures of shallow water and 

 the echinoderms are chiefly characteristic of the 

 deeper portions of the oceans. We also see it in com- 

 paring the lamp-shells or brachiopods of the present 

 day with those that flourished in the palaeozoic seas. 

 But however the relative numbers in the different 

 groups may vary, each major group will always be 

 present, and not only present but well represented. 



All of the other major groups of animals have some 

 special economic niche wherein they are safe from 

 direct competition by creatures belonging to any of 

 the other groups. The only possible exception to 

 this rule is furnished by the graptolites, which have 

 been long extinct. But whether these were of them- 

 selves a major group, or a division of the coelenterates, 

 or a division of the cestodes, we do not really know. 



Now these special niches must have been present 

 at the time when life on earth first began. None of 

 them can be assumed to have been of recent origin. 

 We recognize the fact that wherever there is a res- 

 ervoir of food, permanent or temporary, capable of 

 supporting animal life, there animal life of some sort 

 or other will be found. If it is reasonable to apply a 

 knowledge of geological processes based on the pres- 

 ent to an interpretation of the geological processes of 



