136 THE THEORIES OF EVOLUTION 



temia Salina), there are 168 granular chromosomes, 

 each of which must be regarded as an id. In the ma- 

 jority of living beings, however, the chromosomes are 

 shaped like tiny rods which are composed of a series 

 of granules. Each of these granules is to be regarded 

 as an id, and the chromosomes constitute units of a 

 higher order called "idants." Every id is composed 

 of a particle of germ plasm containing everything 

 which is necessary to make up a complete individual; 

 the ids are therefore to a certain extent what Weis- 

 mann calls primary constituents of individuals (Per- 

 sonen-Anlagen) . 



Here we must point out a similarity (recognised 

 by Weismann) between the Weismannian theory 

 of ids and the old evolutionary theory of germ encase- 

 ment. It goes without saying that Weismann does 

 not believe in any "homunculus" and that, while every 

 id contains all that is necessary to make up a complete 

 individual, there is absolutely no likeness between the 

 embryonic shape of one of its parts and the fully de- 

 veloped part. 



As an organism is composed of dissimilar parts, 

 Weismann is led to conclude that this dissimilarity 

 must already exist in the id wherein the parts orig- 

 inate. Therefore the id must be made up of even 

 smaller units whose co-operation is necessary to form 

 the individual and every one of which controls one of 

 its organs. In other words, every part of the individ- 



