42 2 THE GERM-PLASM 



character are represented in different regions of the body, affords 

 US a principle by which we can understand the unequal effect 

 produced by similar modifying influences upon various regions 

 of the body. 



Even though it can no longer be doubted that climatic and 

 other external influences are capable of producing permanent 

 variations in a species, owing to the fact that, after acting uni- 

 formly for a long period, they cause the first slight modifications 

 of certain determinants to increase, and gradually affect the less 

 changeable variants of the determinants also, the countless 

 majority of modifications is not due to this cause, but to the 

 processes of selection. The question then arises as to the origin 

 of variations which are sufficiently considerable for natural selec- 

 tion to act upon them. External influences at first produce only 

 very slight fluctuations in the determinants — presumably not 

 only in some, but in all ; a continual supply of the most minide 

 variations of the different determinants will thus always be 

 present. 



I do not, however, believe that variations, as we perceive 

 them, are the direct result of these minute fluctuations on the 

 part of individual determinants : they can only be produced by 

 the accumulation of a large number of fluctuations of this kind. 

 This is an immediate consequence of the theory. Since the germ- 

 plasm consists of many ids, each of which contains the same 

 number of homologous determinants, and as, moreover, any 

 character is the result of the interaction of all its homologous 

 determinants, the variation of a single determinant would be 

 imperceptible : a character can only be modified to an appreci- 

 able extent when a majority of the determinants are equivalent, 

 or at any rate are similarly modified. 



In my opinion, a variation of this kind is produced by 

 solitary homodynamous determinants in different ids and in- 

 dividuals being brought together in one germ-plasm by means 

 of the processes of • reducing division ' and amphimixis, so that 

 they can thus form a majority. 



I will illustrate this by a simple example. A small brown 

 European butterfly, Lyccrna agestis, Hb., has a small black 

 spot formed by a few scales in the centre of the wing. Let us 

 suppose that this spot is controlled by a single determinant F, 

 and that the germ-plasm in this species contains a hundred ids, 

 and consequently a hundred determinants F ; and that, in 



