LAWS OF VARIATION 219 



my view: I look at all the species of the same genus 

 as having as certainly descended from a common pro- 

 genitor as have the two sexes of any one species. Con- 

 sequently, whatever 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 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 

 are possessed by all the species; — 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 their great difference in closely allied species; — that 

 secondary sexual and ordinary specific differences are 

 generally displayed in the same parts of the organization 

 — are all principles closely connected together. All being 

 mainly due to the species of the same group being the 

 descendants of 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 



