446 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



the walls of the oviducts or uterus has determined the 

 shape of the placenta ; and that the invagination of the 

 embryo which resulted in the development of the am- 

 nion is a result of gravitation. While these facts have 

 an important bearing on the study of inheritance, they 

 have but a collateral relation to evolution ; since the 

 embryo, whether in utero or in ovo, has little oppor- 

 tunity of experiencing the external influences which are 

 only possible at later periods of life. It is during ado- 

 lescence that the normal activities of maturity, except 

 reproduction, are first practised, whether inherited or 

 learned for the first time. The superior capacity of the 

 adolescent stage for acquisition in all directions is well 

 known, and it is reasonable to suppose that since growth 

 is not completed, changes in its details can be most 

 readily introduced. It is to this period of life then that 

 we must look for the effective influence of the factors 

 of evolution in the acquisition of new characters of the 

 soma. And if the nervous, muscular and other tissues 

 react at this period most readily to external stimuli, 

 it is to be supposed that the developing reproductive 

 cells possess the same characteristic, and record in 

 their molecular movements the influences which are 

 experienced by the entire body. Such influences on 

 the reproductive cells, repeated millions of times from 

 generation to generation, must produce a definite effect 

 on them, in spite of the conservatism which their com- 

 parative isolation imposes on them. 1 



The transmission of acquired characters is evi- 

 dently accomplished during the adult period. While 

 the influence on the soma is greatest during ado- 

 lescence, the influence on the germ- plasma is prob- 

 ably important during maturity, because habits formed 



^American Naturalist, December, 1889, "On Inheritance in Evolution." 



