THE FACTORIAL HYPOTHESIS 225 



to Weismann. Today, however, we have advanced 

 beyond Weismann in this respect, and may more 

 specifically interpret our numerical results of inde- 

 pendent segregation, linkage, and even crossing over 

 on the basis of a chromosome mechanism. More- 

 over, the new facts have given us ideas very different 

 from those of Weismann regarding the arrangement 

 of the factors in the chromosomes and the way in 

 which the characters of an individual are determined 

 by the chromosomal factors. 



In the last edition of his Vortrsege ueber Descen- 

 denztheorie (3d edition, 1913) Weismann modifies 

 his earlier views in regard to the factorial nature of 

 the chromosomes so that his conception of the germ 

 plasm is brought into harmony with the Mendelian 

 theory of heredity. Formerly he had supposed that 

 the chromosomes are all alike, or nearly alike, in so 

 far as each one carries a full assortment of "ids." 

 Each id, in itself, represented the full complement of 

 all the factors that go to make up the organism. 

 But since the results of Mendelian heredity show that 

 all sorts of characters, however trivial, may be segre- 

 gated independently (which would not be the case, 

 if, as Weismann formerly supposed, all the heredi- 

 tary characters are carried by each chromosome), 

 it follows that the chromosomes must be bearers of 

 part ids (Theil Ids). 



Weismann still adheres nevertheless to his mosaic 

 theory of development, but as before stated the 

 modern work on development does not support this 

 interpretation of development. His view assumes 



