THE FACTORIAL HYPOTHESIS 279 
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 Vortreege ueber Descen- 
denztheorie (8d 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 
