194 HABIT ATs^D INTELLIGENCE. [chap. 



No species is able to go on doing so for an indefinite number of 

 ^^d finite^ generations.^ It appears probable that any race whatever 

 time will at last lose its vigour, and die out, if its life is not, as 



Texuar it were, renewed by the mixture of germinal matter from 

 reproduc- another source. 



y .' But, whatever may be the ground of the law of sexuality, 



bUity pro- there can be no doubt of the beneficial effect both of slight 

 ^Jm ^^ changes in the conditions of life, and of mixture of nearly 

 change allied though distinct races ; and it is also true, that changes 

 ditions, in the circumstances of life, and mixture of races, tend to 

 mixture pi"omote variability .^ 



of races. I am now speaking of spontaneous variation. This, as 



I have mentioned at the beginning of this chapter, is a 



distinct thing from functionally-produced change; and I 



propose to call the tendency of a race to vary spontane- 



Varia- ously, variability, and its aptitude for acquiring function- 



^ili*7J^'^'^^ ally-produced changes, modifialility. These two properties 



ability. are different ; but there is, I think, a good deal of reason 



for believing that they are, to a great extent, found together 



— at least among the highest animals. The wonderful 



modifiability of the instinctive faculties of the dog, which 



has enabled him to acquire totally new instincts under 



domestication, must, I think, be in some way connected 



with the variability of his form, which has given origin 



to such different races as the greyhound and the 



terrier. 



Sponta- Spontaneous variations may be defined as those differ- 



^lion h^^ ences of offspring from parent, and of the offspring of the 



far con- same parents from each other, for which no reason can be 



■with the assigned. These do not come under the law of habit, m 



laws of ^]^g^|. ^i-^gy do not originate, in virtue of that law, by the 



repetition of actions ; but they do come under the law of 



habit, in that they are instances of the variability of habit. 



1 Darwin's Origin of Species, p. 109. 



2 See Dar-nin's Origin of Species, p. 21. The statement that changes in 

 the conditions of life tend to promote variability is repeated several times 

 throughout the work, and is illustrated by the examples of various 

 domestic animals and cultivated plants. 



