158 LAWS OF VAKIATION. Chai>. V. 



order to fit the several species to their several places 

 in the economy of nature, and likewise to fit the two 

 sexes of the same species to each other, or to fit the 

 males and females to different habits of life, or the 

 males to struggle with other males for the possession 

 of the females. 



Finally, then, I conclude that the greater variability 

 of specific characters, or those which distinguish species 

 from species, than of generic characters, or those which 

 the species possess in common; — that the frequent ex- 

 treme variability of any part which is developed in a 

 species in an extraordinary manner in comparison with 

 the same part in its congeners ; and the not great 

 degree of variability in a part, however extraordinarily 

 it may be developed, if it be common to a whole group 

 of species ; — that the great variability of secondary 

 sexual characters, and the great amount of difference in 

 these same characters between closely allied species ; — 

 that secondary sexual and ordinary specific differences 

 are generally displayed in the same parts of the organ- 

 isation, — are all principles closely connected together. 

 All being mainly due to the species of the same group 

 having descended from a common progenitor, from 

 whom they have inherited much in common, — to parts 

 which have recently and largely varied being more 

 likely still to go on varying than parts which have long 

 been inherited and have not varied, — to natural selec- 

 tion having more or less completely, according to the 

 lapse of time, overmastered the tendency to reversion 

 and to further variability, — to sexual selection being 

 less rigid than ordinary selection, — and to variations in 

 the same parts having been accumulated by natural 

 and sexual selection, and thus adapted for secondary 

 sexual, and for ordinary specific purposes. 



