HEREDITY. 447 



during adolescence are now practised with especial 

 energy and frequency. The influence on the constantly 

 renewed germ-plasma is correspondingly greater, and 

 transmission is of course more certain. Some charac- 

 ters seem to have been mainly acquired during matur- 

 ity. Such is the permanent dentition of the higher 

 Mammalia, which does not appear until or after ma- 

 turity. In this case the influence of use on the germ- 

 plasma must be more energetic than that on the soma. 

 It is, however, not unlikely that the fundamental char- 

 acters of mammalian dentition were laid during ado- 

 lescence by direct influence on the temporary dentition. 

 The tritubercular molar was established at that time 

 and owes its present existence to inheritance. Only 

 the sectorial and lophodont types have been added 

 since the extensive development of the milk dentition 

 in geologic time. 



The chief source from which acquired characters 

 are introduced into the germ-plasma, and hence into 

 the soma of the next generation, is probably the sper- 

 matozooid, since it is endowed with a greater kinetic 

 energy than the ovum. The latter furnishes nutritive 

 material for the supply of the needs of growth. That 

 the male is the chief source of variation is also indi- 

 cated in the numerous cases when he is more active 

 than the female, and hence more capable of supplying 



the stimulus of use. 



The manner in which influences which have af-' 



fected the general structure are introduced into the 

 germ-cells remains the most difficult problem of biol- 

 ogy. For its explanation we have nothing as yet but 

 hypotheses. The one which has seemed to me to be 

 the most reasonable belongs to the field of molecular 

 physics, and it must be long before it is either proved 





