280 THE FACTORIAL HYPOTHESIS 



disintegration of the germ plasm when the body cells 

 are produced in order to account for the localization 

 of characters; the other view, following the experi- 

 mental results and microscopical observations, as- 

 sumes, so far as the chromosomal materials are con- 

 cerned, that all of the hereditary factors are present 

 in every cell in the body. This view is essentially 

 that proposed by DeVries in his book on Intracellular 

 Pangenesis. The cause of the differentiation of the 

 cells of the embryo is not explained on the factorial 

 hypothesis of heredity. On the factorial hypothesis 

 the factors are conceived as chemical materials in the 

 egg, which, hke all chemical bodies, have definite 

 composition. The characters of the organism are 

 far removed, in all hkehhood, from these materials. 

 Between the two lies the w^hole world of embryonic 

 development in which many and varied reactions 

 take place before the end result, the character, emerges. 

 Obviously, however, if every cell in the body of one 

 individual has one complex, and every cell in the 

 body of another individual has another complex that 

 differs from the former by one difference, we can treat 

 the two systems as two complexes quite irrespective 

 of what development does so long as development is 

 orderly. 



It is sometimes said that our theories of heredity 

 must remain superficial until we know something 

 of the reactions that transform the egg into the adult. 

 There can be no question of the paramount impor- 

 tance of finding' out what takes place during develop- 

 ment. The efforts of all students of experimental 



