198 HABIT AND INTELLIGENCE. [cHAP. 



having, as I have already expressed it, acquired a habit of 

 varying. Thus, among the domestic races of animals, the 

 most variable characters are those in wliich each breed 

 differs from the other breeds of the same species ; and 

 these are characters which, having arisen by variation 

 since the races were first domesticated, must have varied 

 conspicuously in a time which is very short in comparison 

 with the lifetime of a species. 

 Correla- Certain variations are habitually found together, or, as 



variations. Darwin expresses it, are correlated. The nature of the 

 correlation is sometimes quite unintelligible : thus, per- 

 fectly white cats frequently have blue eyes instead of 

 green, and are deaf^ But in many cases it is to be referred 

 Homo- to the simple and intelligible law, that homologous parts 

 paits vary tend to vary together. Thus, similar characters on the 

 together, opposite sides of the body are so much a matter of course, 

 that any exception to the law is regarded as a monstrosity 

 — as, for instance, when the hands are of unequal length ; 

 and, in the same way, the hands and the feet, which are 

 homologous parts, usually have similar characters. It 

 would be very difficult to find a person with large hands 

 and small feet, or the reverse. 

 Mental Throughout the organic creation some classes of func- 



rl f 



charaoters ^ions appear to be much more variable than others. Mental 

 are more and motor habits are much more variable than formative 

 than for- ones : thus, in man, mental characters are incomparably 

 more variable than bodily ones as between different indi- 

 viduals, and also more modifiable by education and cir- 

 cumstances. The same appears to be true of the domestic 

 races of animals, at least of the more intelligent ones. As 

 instances of the changeableness of purely motor functions 

 may be mentioned the cases of the domestic hen, duck, 

 and goose, which from disuse have nearly lost the power of 

 flight, without the muscles of their wings being diminished 

 in size in any proportionate degree. These are cases of 

 functionally-changed habit. The habit from which the 

 tumbler variety of the pigeon derives its name is a far 

 more extraordinary case, and is certainly unique among 



' Darwin's Origin of Species, p. 12. 



mative 

 ones. 



