312 THE RECAPITULATION THEORY 



entirely omitted, the shell acquiring almost from its 

 earliest commencement the cyclical mode of growth. 

 There is no question here of relative abundance of 

 food yolk, but merely of early or precocious 

 appearance of adult characters. 



The question of the relations and influence of 

 food yolk, involving as it does the larger or smaller 

 size of the egg, is however merely a special side 

 of the much wider question of the nutrition of the 

 embryo, one of the most potent of the disturbing 

 elements affecting development. Speaking gene- 

 rally, we may say that large eggs are more often 

 met with in the higher than the lower groups of 

 animals. Birds and Reptiles are cases in point, 

 and if Mammals do not now produce large eggs, 

 it is because a more direct and more efficient mode 

 of nourishing the young by the placenta has been 

 acquired by the higher forms, and has replaced the 

 food yolk that was formerly present, and is now 

 retained in quantity by Monotremes alone. Molluscs 

 afford another good example, the eggs of Cephalo- 

 poda being of larger size than those of the less 

 highly organised groups. The large size of the 

 eggs of Elasmobranchs, and perhaps that of Cepha- 

 lopods also, may possibly be associated with the 

 carnivorous habits of the animals ; for it is of im- 

 portance that forms which prey on other animals 

 should hatch of considerable size and strength. 



The influence of habitat must also be considered. 

 It has long been noticed as a general rule that 

 marine animals lay small eggs, while their fresh- 

 water allies have eggs of much larger size. The 



