GERMINAL SELECTION 155 



may be taken as certain, that multitudinous inter-relations and 

 influences exist between the elements of the germ-plasm, and that 

 one variation brings another in its train, so that— usually at a very 

 slow rate, that is, in the course of generations and of species-forming, 

 definite variations occur from purely intra-germinal reasons— varia- 

 tions which as far as they remain outside the limits of good or luid 

 may of themselves change the character of a species, but which when 

 they are seized upon by personal selection may, by sifting and com- 

 bination of the ids, be led on to still higher development. 



If we consider further that the variation of a part must depend 

 not only on the quality of the external stimulus but also upon the 

 constitution, the reacting power of the part, we shall understand that 

 similar nutritive variations may cause two different determinants 

 to vary in different ways, and when we reflect that every nutritive 

 change must extend from the point from which it started with 

 diminishing strength in a particular direction, we have a furtlier 

 factor in the variation of determinants and one which influences even 

 similar determinants differently. 



Finally, if we remember that determinants of difterent constitu- 

 tion will also extract different ingredients from the nutritive stream 

 and thus set up in it diflerent kinds of chemical change, tlius causing 

 an altered supply of nutritive substances to flow to tlie neighljour 

 determinants, we get some insight into a very complex and delicate 

 but perfectly definite set of processes, into a mechanism whieli we can 

 certainly only guess at, but whose results lie plainly before us in the 

 spontaneous variations of the organism. We understand in principle 

 the possibility of saltatory variation, as a more or less widespread, 

 more or less marked disturbance of the species-type in this or that 

 group of characters, and we may acknowledge that those 'kaleidoscopic 

 variations ' which Eimer supposed to be the sole basis of the trans- 

 formation of species, and which have been l)rought to the foreground 

 again quite recently by De Vries \ are probably factors in transmuta- 

 tion operative within a limited sphere. 



But we must think of all these struggles and nmtual influencings 

 as taking place on the smallest possible scale, so that it is only by long 

 summation that they can produce any visilJe efiect,and we nmst never 

 forget the essential significance of the plurality of ids, for these 

 'spontaneous' variations may take place in a difterent and (^uite 

 independent manner in each individual id. If this were not so no 

 intervention of personal selection would be possible, natural selection 

 would not exist, and the adaptation of the organism from the single 



1 See end of chap, xxxiii. 



