G ALTON AND STATISTICAL STUDY OF INHERITANCE l6l 



fully coordinated with each other in such a way as to promote the 

 welfare of the species. A generalization which ignores this fact may, 

 while proved by statistics, be untrustworthy as a contribution to our 

 knowledge of inheritance. 



In popular language, specific stability may be said to be due to 

 inheritance, and specific mutability to variation ; but in this connec- 

 tion these words have only a loose meaning. In so far as they con- 

 vey the impression that the stability of species and mutability of 

 species are antagonistic to each other, or are due to two distinct and 

 opposing influences, these terms are unfortunate, for we have good 

 ground for believing that they are only contrasted aspects of the same 

 phenomenon the extermination of certain individual peculiarities, 

 and the preservation of others, by natural selection. 



The older naturalists held that adherence to type is due to some 

 innate principle of specific stability which is an essential and immu- 

 table attribute of each species of living things ; but the accumulation 

 of conclusive evidence of the mutability of species has driven this 

 conception out of the field. Most naturalists now regard the type as 

 nothing but that normal which is most perfectly fitted to the environ- 

 ment, and they hold that it is kept true through the extinction of 

 aberrant individuals by selection. 



According to this view, which seems to be supported by ample 

 evidence, the stability of species is due to survival to the same 

 mechanism which brings about the mutability of species. They hold 

 that neither the stability nor the mutability of species is anything 

 more than the struggle for existence would lead one to expect ; and 

 that which we call inheritance and that which we call variation not 

 two things, but one thing in two points of view. 



Galton is led by his statistical studies of vital characters to a 

 view which bears an odd resemblance to that of the older naturalists ; 

 for, according to him, the principle which results in the permanency 

 of types is quite independent of selection. 



He shows, for example, by the statistical study of stature, that 

 the type of human stature is very constant from generation to gener- 

 ation, although the statistics of marriage show that there is no con- 

 trolling tendency for persons of like stature to marry. He also 

 shows that the children of parents who are both tall or both short do 

 not on the average have the stature of their parents, but are nearer 



