INTERNAL CAUSES OF VARIATION 213 



the physiological units are capable of a cycle of differentiated 

 energies, or else the race is possessed of many kinds of units, 

 each inactive until its turn, then playing its role in suitable 

 order. What then establishes the order of activity and calls 

 out each unit at the proper time ? Herein lies the mystery, and 

 while the physiological unit seems a biological fact, it after all 

 does not solve the mystery of differentiation ; it only pushes the 

 problem one step farther away. 



Unsatisfactory though it is to attempt to solve the mystery 

 of inheritance and differentiation by means even of vital units, 

 still nothing else that has yet been proposed comes nearer 

 satisfying the needs of the case, and we cannot fight off the 

 following convictions, namely : 



1. That there is a material basis not only of life but of racial 

 characters as well, and this material passes to the individual by 

 means of the germ cell. 



2. That the processes of life are essentially chemical. 



3. That if the whole truth could be known, the physiological 

 units of vital activity may not be fundamentally different from 

 atoms, molecules, and radicals actuated by chemical affinity. 

 Broadly speaking, there are suggestive similarities between the 

 chemical behavior of living matter and that of laboratory mate- 

 rial generally, and these similarities are constantly turning up, 

 even where least expected. 



Whatever the truth may be as to the unit of vital activity, of 

 two things we are sure : first, there is a unit of some kind, a 

 center of activity ; and second, it is a chemical material pos- 

 sessed of life. Finally, to be useful, these units must be 

 conceived as capable of absorbing nourishment, and of self- 

 multiplication indefinitely. 



SECTION XVI GERMINAL SELECTION 



The difficulty in seeking causes for inheritance and variation 

 is that we are likely to prove too much. For example, "if suffi- 

 cient plasticity is assumed to fully account for the high degree 

 of variation that often occurs, then variation is sufficiently pro- 

 vided for, but this view makes a thing like inheritance a matter 



