174 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



that it should be a perfect repetition of phytogeny; very much of the 

 long story must of necessity be omitted. 



2. Through all the stages of development the embryo must be 

 rendered able to live and grow and thrive through adaptation to its 

 surroundings and changes in its environment. In some animals 

 development takes place within the body of the mother; in others the 

 embryo is protected by the hard egg-shell, as in birds, while the eggs 

 of certain fishes and many invertebrates float freely in the sea and are 

 almost without protection. Such differences in environment necessi- 

 tate differences in the mode of 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 ontogeny. 



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 hi appearance and structure 

 from its adult parents. Familiar instances of this mode of develop- 

 ment 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 environ- 

 ment and to changes in that environment, exactly as do adults, if they 

 are to survive. In this way many changes are introduced into the 

 ontogeny which can have no phylogenetic significance. It is found in 

 several known instances, that nearly allied species, living under 

 different conditions, 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 6f the 

 southern. In attempting to interpret the meaning of embryological 

 facts, it is thus necessary to distinguish sharply between those features 

 which are derived from a long inheritance, and are therefore called 

 palingenetic, from those which have been secondarily introduced in 

 response to the changing needs of embryonic or larval life. These 

 secondary features are termed cenogenetlc. 



"If we are compelled to admit that cenogenetic characters are 

 intermingled with palingenetic, then we cannot regard ontogeny as a 

 pure source of evidence regarding phyletic relationships. Ontogeny 

 accordingly becomes a field in which an active imagination has full 

 scope for its dangerous play, but in which positive results are by no 

 means everywhere to be obtained. To attain such results, the palin- 



