THEORY OF THE DEVELOPMENT OF ORGANISMS 103 



differentiations of the soma claim our attention and 

 require an explanation in the first instance. The 

 presence of idioplasm in all parts containing the 

 primary constituents does not help us in this 

 respect.' 



With this I cannot agree. Naturally, Naegeli, 

 De Vries, Driesch and I assume that, of the many 

 rudiments present in every cell, only some come to 

 activity in each special case, and that the selection 

 of those that become active is due to causes arising 

 in the course of development. Our conception of 

 the nature of these causes, and of their place of 

 origin, is diametrically opposed to Weismann's. 



Weismann would make the causes of this orderly 

 development of the rudiments reside in the germ- 

 plasm itself; for he considers that to be not only 

 the material but the motive force of the course of 

 development. According to him, every cell must 

 have become what it is, because it was provided 

 only with the definite rudiments assigned it before- 

 hand, according to the plan of the development of 

 the germplasm. 



On the other hand, we regard the development of 

 the rudiments as depending upon motive forces or 

 causes that are external to the germplasm of the 

 ovum, but that none the less arise in orderly 

 sequence throughout the course of the development. 

 The causes we recognise are first, the continual 



O 



changes in mutual relations that the cells undergo 

 as they increase in number by division, and second, 

 the influence of surrounding things upon the 

 organism. 



