434 THE GERM-PLASM 



determinants to accumulate into majorities by mingling the 

 halved germ-plasms of two individuals. By the aid of the 

 ' reducing division " it can also level and equalise matters by 

 fortuitously dispersing the homologously modified determinants 

 of an individual. 



It must not be forgotten that slight primary variations of a 

 determinant need not ahvavs continue in the same direction : 

 influences of nutrition in the reverse direction will frequently 

 cause them once more to disappear. Only after a determinant has 

 been modified to a considerable degree by the action of a uniform 

 influence during a long period, and the determinants of many 

 ids have become similarly and simultaneously modified, can a 

 variation become visible — after being first accumulated by 

 amphimixis. And even then it by no means forms a permanent 

 specific character, for the question as to whether it will or will 

 not give rise to one is decided by natural selection. 



Thus several powerful influences prevent the constant varia- 

 tion of the specific type. 



An answer to the question as to wJiat variations the idio- 

 plasm undergoes in the transfortnation of species will be found 

 in the chapter on reversion, and it will here only be necessary to 

 summarise what I have already said. 



The transformation of species is due to the variation of some, 

 and frequently even of most, of the determinants. Many species 

 do not possess a single character which resembles that of an 

 allied species, and in this case all the determinants must be 

 diflferent. But this only implies that all the determinants a — x 

 have been modified /;/ tJie majority of ids ; a minority of the 

 latter will contain unmodified ancestral determinants. As the 

 transformation of a species proceeds, the number of modified 

 determinants increases together with the number of ids in 

 •which they occur. Nevertheless the dominating principle of 

 selection only permits the transformation of all the ids to occur 

 very gradually, so that the germ-plasm of a young species may 

 often contain completely unmodified ids of the ancestral species; 

 and even older species may contain solitary groups of unmodi- 

 fied determinants in many of their ids. This, and this alone, 

 renders reversion possible. 



It has been recently maintained that, as a consequence of 

 my theory, I must adopt one of two alternatives, and assume 

 cither that the germ-plasm of the higher animals consists of ids 



