316 THE THEORY OF THE GENE 



changes due to a mutated gene that affect physiological 

 processes and reactions may frequently be accompanied 

 by alterations in external structural characters. If these 

 physiological changes are of a kind to better adjust the 

 organism to its environment, they may be expected to 

 persist, and, at times, lead to the survival of new types. 

 These types may then differ from the original type in 

 superficial characters that are constant but trivial in 

 themselves. Since many species differences appear to be 

 of this kind, it is plausible to interpret their constancy 

 as due not to their own survival value, but rather due to 

 their relation to some other deeply seated character that 

 is important for the welfare of the species. 



In the light of what has just been said we can give a 

 reasonable explanation of the differences that follow 

 when a mutant change involves a whole chromosome (or 

 part of one) and when only a single gene is involved. The 

 former change adds nothing intrinsically new to the 

 situation. More or less of what is already present is in- 

 volved in the change, and the effects are small in degree 

 but involve a large number of characters. The latter 

 change — mutation in a single gene — may also produce 

 widespread and slight effects, but, in addition, it often 

 happens that one part of the body is changed to a strik- 

 ing degree along with other changes less striking. This 

 latter kind of change, as I have said, supplies materials 

 favorable for genetic study ; these have been widely uti- 

 lized. Now it is these mutational changes that have occu- 

 pied the forefront of genetic publication, and have given 

 rise to a popular illusion that each such mutant character 

 is the effect of only one gene, and by implication to the 

 fallacv, more insidious still, that each unit character has 

 a single representative in the germ material. On the con- 

 trary, the study of embryology shows that every organ 

 of the body is the end-result, the culmination of a long 



