4 THE GERM-PLASM 



one generation to the next : they may, under certain conditions, 

 become active, and thus again bring into existence ancestral 

 traits which had disappeared in the parents. 



This is, in brief, the theory of pangenesis. It does not take 

 into account the physical nature of the gemmules. They are 

 capable of multiplying, and do so continually : but the question 

 as to whether they have any definite arrangement, and if so, 

 what the nature of that arrangement is, is not touched upon ; 

 nor is any mention made of the causes and mechanism by which 

 it comes about that they are always in the right place and 

 develop into cells at the right time. 



I do not say this by any means as a reproach, but only to 

 bring out clearly the speculative character of the whole hypothe- 

 sis. Darwin did not go on to inquire whether all these assump- 

 tions were possible : he only asked what it was necessary to 

 assume in order to explain this or that fact of heredity, without 

 troubling himself to consider whether the assumption were borne 

 out by facts or not. And he was right in doing so, for at the 

 time w^ien he propounded his hypothesis it was not possible to 

 found any theory of heredity on the only sound basis, — that of 

 a knowledge of the most minute cell-structure. I have already 

 pointed out how extremely important and fruitful his theory 

 of pangenesis has been : it drew attention for the first time to 

 all the phenomena that needed explanation, and showed what 

 assumptions must be made in order to explain them. 



It will be shown later on that, in spite of the fact that a con- 

 siderable number of these assumptions are untenable, a part of 

 the theory still remains which must be accepted as fundamental 

 and correct, — in principle at any rate, — not only now, but also 

 for all time to come. I refer to the most general portion of 

 these assumptions only, namely, that presupposing the existence 

 of material particles in the germ which possess the properties of 

 the living being, and each of which is to be regarded as the 

 primary constituent (' Anlage ') of one portion of the organism. 

 I must honestly confess to having mentally resisted this fun- 

 damental point of the Darwinian doctrine for a long time. It 

 appeared almost impossible to me that such an enormously large 

 number of individual primary constituents as we must suppose 

 to exist, according to Darwin's view, could be contained in the 

 minimum of substance which, as will be shown hereafter, we have 

 to regard as the actual bearer of heredity. I tried in several 



