170 



EVOLUTION AND ANIMAL LIFE 



If this is true, it would appear that nervous overstrain of 

 the parent is unfavorable to normal nerve development of the 

 offspring. This would be apparently a case of transmission of 

 parental conditions, as above indicated, and not one of true 

 heredity. 



It may be conceived that, at the moment of impregnation, 

 the resultant germ cell is sexless. It begins its development at 

 once, and, in the higher animals, turns very soon toward the 

 formation of those structures which distinguish the one sex or 

 the other. Each individual ultimately becomes either male or 

 female. Relatively few animals, and those among the lower 



Fig. 102. — Limb skeletons of extinct and living animals, showing the homologous 

 bones: 1, salamander; 2, frog; 3, turtle; 4, Aetosaurus; 5, Plesiosaurus; 6, Ichthyo- 

 saurus; 7, Mososaurus; 8, duck. 



forms, are ever really hermaphrodite, or representative of both 

 sexes at once. 



Among the invertebrate animals the numerical relations of 

 the sexes are subject to great variation. Among vertebrates, 

 in general, the sexes are practically equal in number, as is shown 

 by count of large series of individuals. This is true whether the 

 species be monogamous, polygamous, or promiscuous in its sex 

 relations. It is therefore apparent that the sex tendencies in 



