^^'"^ THE NEW EVOLUTION Wi 



the past, it is no less reasonable to do the same with 

 our knowledge of zoology. If we do this we must 

 believe that all the various niches must have been 

 occupied at the very first instead of serially. 



Still another aspect of the problem demands con- 

 sideration. No animal type can exist of itself alone. 

 In the first place, it must be supplied with food 

 through the medium of coexisting plants. In the 

 second place, it must be held in check so as to avoid 

 the danger of increasing to such an extent as to destroy 

 its food supply and thus bring about its own ex- 

 termination. 



The necessary check on the excessive increase in 

 the numbers of any type of animal is provided by pre- 

 dacious animals, by internal and external parasites, 

 and" by various types of animal feeding plants, princi- 

 pally bacteria and fungi. 



We recognize the necessity for an intricate and 

 delicate balance between the various types among the 

 animals and plants of the present day. Is there any 

 reason why in the remotest past a similar balance 

 should not have been as necessary? 



It is probably not without significance that no 

 major group of animals includes exclusively plant- 

 eating species, though in some of the major groups all 

 of the included species are carnivorous; and besides 

 these in several large groups, such as the cestodes or 

 tapeworms (figs. 8x, 83, p. 161) and the spiny-headed 

 worms, all of the included forms are parasitic. 



We cannot deny that at the time when the first 

 appearance of animal life was possible the poten- 



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