268 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



during the life of an individual, cannot be transmitted ? We are 

 clearly compelled to find some other source of hereditary in- 

 dividual differences, or the theory of natural selection would collapse, 

 as it certainly would if hereditary individual variations did not 

 exist. If, on the other hand, acquired differences are transmitted, 

 this would prove that there must be something wrong in the 

 theory of the continuity of the germ-plasm, as above described, and 

 in the non-transmission of acquired characters which results from 

 this theory. But I believe that it is possible to suggest that the 

 origin of hereditary individual characters takes place in a manner 

 quite different from any which has been as yet brought forward. 

 To explain this origin is the task which I am about to undertake 

 in the following pages. 



The origin of individual variability has been hitherto represented 

 somewhat as follows. The phenomena of heredity lead to the 

 conclusion that each organism is capable of producing germs, from 

 which, theoretically at least, exact copies of the parent may arise. 

 In reality this is never the case, because each organism possesses 

 the power of reacting on the different external influences with 

 which it is brought into contact, a power without which it 

 could neither develope nor exist. Each organism reacting in a 

 different way must be to some extent changed. Favourable nutri- 

 tion makes such an organism strong and large ; unfavourable nu- 

 trition renders it small and weak, and what is true of the whole 

 organism may also be said of its parts. Now it is obvious that 

 even the children of the same mother meet with influences different 

 in kind and degree, from the very beginning of their existence, so 

 that they must necessarily become unlike, even if we suppose them 

 to have been derived from absolutely identical germs, with precisely 

 the same hereditary tendencies. 



In this manner individual differences are believed to have been 

 introduced. But if acquired characters are not transmitted the 

 whole chain of argument collapses, for none of those changes which 

 are caused by the conditions of nutrition acting upon single parts 

 of the whole organism, including the results of training and of the 

 use or disuse of single organs, none of these changes can furnish 

 hereditary differences, nor can they be transmitted to succeeding 

 generations. They are, as it were, only transient characters as far 

 as the species is concerned. 



