396 THE EVOLUTION THEORY 



the cause of the growth, at definite places, of claws and adhesive lobes 

 with all their characteristically placed hairs. 



We require to assume that each of the cells composing the 

 primary rudiment of the limb possessed a determining power which 

 made it grow and multiply under the given conditions of nutrition 

 and pressure in a prescribed manner and at a prescribed rate ; and 

 we must make the same assumption in regard to all the daughter 

 and grand-daughter- cells, and so on. The strictest regulation of the 

 power of multiplication of each of the implicated cells is a necessary 

 condition of the constant production of the same two claws and 

 adhesive lobes, the same form of tarsal joint, the same regular 

 covering of hair, and so on. This exact determination of the cells 

 can only take place through material vital particles, and it is these 

 which I call determinants. 



I have already said so much about the assumed ' determinants ' 

 of the germ-plasm that it might perhaps be supposed that we have 

 now exhausted the topic ; but the assumption of such ' primary 

 constituents ' is so fundamental, not only for my own germ-plasm 

 theory of to-day and to-morrow, but also — unless I am much mis- 

 taken — for all future theories of development and inheritance. In 

 point of fact, the conception of determinants has as yet penetrated 

 so little into the consciousness of biologists, that I cannot remain 

 content with what I have already said, but must endeavour to test 

 and to corroborate my thesis by additional illustrations. 



As far as I am aware, only a few zoologists have expressly and 

 unconditionally agreed with the assumption of determinants ; on the 

 other hand, several biologists have rejected it as fanciful and un- 

 tenable, while others have set it aside as a useless playing with ideas. 

 The last, I am inclined to believe, have not taken the trouble to 

 think out what the idea is. It has even been objected that there can be 

 no determinants because we can see nothing of them, and that they 

 must therefore be pure figments of the imagination, invented to 

 explain facts which could be explained much more easily and simply 

 in some other way. From the very first I have stated emphatically 

 that they have not been, and never will be seen, because they lie 

 far below the limit of visibility, and thus can at best only become visible 

 when they are collected in large aggregates like chromatin granules. 

 Nor have I any objections to make if any one chooses to describe all 

 the details of their activity as mere hypotheses, such, for instance, 

 as their distribution during development, their ' maturation,' their 

 migration from the nucleus, and the manner in which they control 

 the cell. All this is really an imaginative picture which may be 



