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 

 specialized 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 satisfactory ; 

 no one of them can be regarded as even the beginning of a solution or 

 as a secure foundation from which a complete solution may be 

 expected in the future. Neither Hackel's 1 , 'Perigenesis of the 

 Plastidule,' nor Darwin's 2 ' Pangenesis,' can be regarded as such a 

 beginning. The former hypothesis does not really treat of that 



1 Hackel, 'TJeber die Wellenzeugung der Lebenstheilchen etc.j' Berlin, 1876. 

 a Darwin, ' The Variation of Animals and Plants under Domestication,' vol. ii. 

 1875, chap, xxvii. pp. 344-399. 



