EVIDENCE FROM EMBRYOLOGY 59 



development, while the presence or absence of a 

 large amount of inert food-material, or yolk, exerts 

 a great influence in determining the steps of on- 

 togeny. 



(3) Many animals pass through a larval stage of 

 development, in which the immature young leads 

 an independent and self-sustaining existence, during 

 which it is very different in appearance and structure 

 from its adult parents. Familiar instances of this 

 mode of development are to be found in the tadpole, 

 which is the larva of the frog, and the caterpillar the 

 larva of a butterfly. Larvae are fully subject to 

 the struggle for existence and must adapt themselves 

 to their environment and to changes in that environ- 

 ment, exactly as do adults, if they are to survive. In 

 this way many changes are introduced into the 

 ontogeny which can have no phylogenetic signifi- 

 cance. It is found in several known instances, that 

 nearly allied species, living under different condi- 

 tions, have quite different modes of ontogeny, though 

 their ancestral history must have been substantially 

 identical. In one and the same species of marine 

 worms, for example, which inhabits both the warm 

 Mediterranean and the cold waters of the North 

 Sea, the larva of the northern form is quite distinct 

 from that of the southern. In attempting to in- 

 terpret the meaning of embryological facts, it is thus 

 necessary to distinguish sharply between those fea- 

 tures which are derived from a long inheritance, 

 and are therefore called paling 'enetic, from those 



