LAWS OF VARIATION 143 



descended from the same progenitor, as have the two 

 sexes of any one of the species. Consequently, what- 

 ever part of the structure of the common progenitor, 

 or of its early descendants, became variable ; variations 

 of this part would, it is highly probable, be taken 

 advantage of by natural and sexual selection, in 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 slight 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 organisa- 

 tion, — 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 

 selection having more or less completely, according 

 to the lapse of time, overmastered the tendency to 

 reversion and to further variability, — to sexual selec- 

 tion being less rigid than ordinary selection, — and to 

 variations in the same parts having been accumulated 

 by natural and sexual selection, and having been thus 

 adapted for secondary sexual, and for ordinary specific 

 purposes. 



