IV. 



THE CONTINUITY OF THE GERM-PLASM 

 AS THE FOUNDATION OF A THEORY 



OF HEREDITY. 



Introduction. 



When we see that, in the higher organisms, the smallest 

 structural details, and the most minute peculiarities of bodily 

 and mental disposition, are transmitted from one generation to 

 another ; when we find in all species of plants and animals 

 a thousand characteristic peculiarities of structure continued 

 unchanged through long series of generations ; when we even 

 see them in many cases unchanged throughout whole geological 

 periods ; we very naturally ask for the causes of such a striking 

 phenomenon: and enquire how it is that such facts become 

 possible, how it is that the individual is able to transmit its 

 structural features to its offspring with such precision. And 

 the immediate answer to such a question must be given in the 

 following terms : — ' A single cell out of the millions of diversely 

 differentiated cells which compose the body, becomes special- 

 ized as a sexual cell ; it is thrown off from the organism and 

 is capable of reproducing all the peculiarities of the parent 

 body, in the new individual which springs from it by cell- 

 division and the complex process of differentiation.' Then the 

 more precise question follows : ' How is it that such a single 

 cell can reproduce the tout ensemble of the parent with all the 

 faithfulness of a portrait ? ' 



The answer is extremely difficult ; and no one of the many 

 attempts to solve the problem can be looked upon as satis- 

 factory; no one of them can be regarded as even the beginning 

 of a solution or as a secure foundation from which a complete 



