1889.] On Inheritance in Evolution. 1067 
suited in the development of the amnion is a result of gravitation. 
While these facts have an important bearing on inheritance, they 
have but a collateral relation to evolution; since the embryo, 
whether in utero or in ovo, has little opportunity of experiencing 
the external influences which are only possible at later periods of 
life. It is during adolescence that the normal activities of maturity 
except reproduction, are first practiced, 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 stimmuli, it is to be supposed 
that the developing reproductive cells possess the same character- 
istic, 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 coniparati\'e isolation imposes on 
them. It is difficult to sec how it can be otherwise in view of 
the evident mechanical origin oi the characters which have suc- 
ceeded each other as the steps of the evolution of vegetable and 
animal types. 
The transmission of acquired characters is evidently principally 
accomplished during the adult period. While the influence on 
the soma is greatest during adolescence, the influence on the 
germ-plasma is greatest during maturity. Here habits formed 
during adolescence are practiced with especial energy and fre- 
quency. The influence on the constantly renewed germ-plasma 
is correspondingly greater, and transmission is of course more cer- 
tain. Some characters seem to have been mainly acquired dur- 
ing maturit)^ Such is the permanent dentition of the higher 
Mammalia, which does not appear until or after niatunt)-. In 
this case the influence of use on the germ-plasma must be more 
energetic than that on the soma. It is howe\'er not unlikely that 
