270 THE FACTORIAL HYPOTHESIS 
and that, in consequence, many factor differences 
may occur which will, in turn, cause the character 
differences in question. Secondly, Bateson argues 
that we should expect these irregularities to occur in 
the segregation of character-factors during germ-cell 
formation, because we find irregularities in the seg- 
regation of factors during development. Appar- 
ently Bateson holds the view that differentiation 
of characters is the result of sorting out of factors 
in the somatic divisions; in other words, he adopts 
Weismann’s theory of embryonic development. Lo- 
calization of factors is inferred from localization of 
characters. Hence his employment of the idea 
chiefly when patterns are involved. The conclusion 
to which most modern students of experimental 
embryology have arrived, a conclusion based on a 
considerable body of evidence, is that differentiation 
is not a consequence of sorting out of the hereditary 
(genetic) materials. This conclusion is not con- 
sidered or else is ignored by Bateson in this argument. 
5. The confusion of character with factor is nowhere 
more apparent than in the well-known presence and 
absence hypothesis, and since this hypothesis has 
been so widely employed in Mendelian literature it 
calls for somewhat more extended analysis. The 
hypothesis was first proposed to explain the inherit- 
ance of combs in poultry (Fig. 64). Rose comb by 
single comb gives in F, three rose to one single; pea 
comb to single gives in F, three pea to one single. 
When rose is bred to pea a new type of comb, called 
walnut, appears, and in F, there are nine walnut: 
