CHAPTER II 

 PARTICULATE THEORIES OF HEREDITY 



THE evidence given in the last chapter led to the 

 conclusion that there are hereditary units in the 

 germinal material that are, to a greater or less 

 extent, independently sorted out between successive gen- 

 erations of individuals. Stated more accurately, the inde- 

 pendent reappearance in later generations of the charac- 

 ters of two individuals combined in a cross can be 

 explained by the theory of independent units in the 

 germinal material. 



Between the characters, that furnish the data for the 

 theory, and the postulated genes, to which the characters 

 are referred, lies the whole field of embryonic develop- 

 ment. The theory of the gene, as here formulated, states 

 nothing with respect to the way in which the genes are 

 connected with the end-product or character. The absence 

 of information relating to this interval does not mean 

 that the process of embryonic development is not of in- 

 terest for genetics. A knowledge of the way in which the 

 genes produce their effects on the developing individual 

 would, no doubt, greatly broaden our ideas relating to 

 heredity, and probably make clearer many phenomena 

 that are obscure at present, but the fact remains that the 

 sorting out of the characters in successive generations 

 can be explained at present without reference to the way 

 in which the gene affects the developmental process. 



There is, nevertheless, a fundamental assumption im- 

 plied in the statement just made, viz., that the develop- 

 mental process follows strictly causal laws. A change in a 



