282 SOCIAL HEREDITY AND SOCIAL EVOLUTION 



Moreover, instincts, which represent phases of 

 evolution seemingly radically different from anat- 

 omy, are in reality based upon the structure of the 

 nervous system. What can seem more unlike than 

 structure and instincts, which are habits of action? 

 But when we come to analyze these instincts we learn 

 that, after all, they are only outward expressions of 

 the structure of the nervous system, and as such they 

 are inherited and not learned. 



That this is a correct method of viewing the 

 problem may be rendered apparent by a considera- 

 tion of the conditions under which instincts appear 

 in the individual. If the egg of one of the lower 

 animals be removed entirely from contact with its 

 parents, and if the young be reared without any 

 knowledge of the habits or instincts of its parents, 

 the animal will universally pass over the same line in 

 its development as did its parents, and will develop 

 instincts practically identical with those of the 

 animal from which it sprang, even though no inter- 

 course has taken place between the parents and the 

 offspring since the production of the egg. Many a 

 bird leaves its eggs to be hatched by the heat of the 

 sun, and yet all the young develop independently the 

 instincts of their parents. The duck hatched in the 

 incubator goes to the water while the chick avoids it. 

 The fish commonly leaves its eggs to be hatched by 

 the heat of the water, and yet the young, who have 

 never seen their parents, develop their instincts. The 

 young salmon seeks the ocean and later comes back 

 to the river for spawning, just as did its parents. 

 The same would undoubtedly be true of mammals if 

 it were possible to separate the offspring from the 

 parents. This is not so easy with mammals as with 



