242 PHYSICAL BASIS OF HEEEDITY 



is that if differences are present at the beginning, the end 

 product is expected to be correspondingly different. So 

 much is clear. But why, it may still be asked, are not two 

 organisms that are diff'erent at the start, if only in some 

 one difference, different later in every part, rather than 

 in only some one small part such as in a red or in a white 

 eye. The answer is, of course, that the first difference 

 was such that it affected principally a particular process, 

 viz., the formation of the red pigment of the eye, and to 

 a less degree, or not at all, other chemical processes. This 

 seems to me an entirely consistent view. 



Perhaps the difficulty in accepting the particulate 

 theory lies in the erroneous idea that the specific effect 

 comes into action only at the moment when the red pig- 

 ment is about to form. But no one has, so^ far as I know, 

 made such a claim. It may be true, but it has not been 

 proven, and is moreover not in any way essential to the 

 assumption of the particulate theory. On the contrary, 

 as our knowledge of Mendelian heredity has increased 

 many cases have been found where a special factor-differ- 

 ence affects not only one part of the body but many parts. 

 It is true that the particulate theory as held at one time 

 by Eoux and for a long time by Weismann was used to 

 explain the differentiating changes in the segmenting 

 egg and embryo in the sense that development was looked 

 upon as a process that resulted immediately in the sorting 

 out of the inherited chromosomal particles to the differ- 

 ent parts of the organism. Differentiation resulted in the 

 sorting out of particular genes to particular groups of 

 cells whose development they controlled. But the cyto- 

 logical evidence in regard to the chromosomes gave no 

 evidence in support of the view, and the evidence from 

 the experimental study of embryology seemed to entirely 

 disprove any such basis for the developmental phe- 

 nomena. In fact, Eoux himself abandoned this view 

 in the light of the brilliant experiments of Driesch and 

 of other embryologists. 



