SPECIFIC CHARACTERS HIGHLY VARIABLE 167 



the two sexes of the same species. Sir J. Lubbock has re- 

 cently remarked, that several minute crustaceans offer ex- 

 cellent illustrations of this law. "In Pontella, for instance, 

 the sexual characters are afforded mainly by the anterior 

 antennas and by the fifth pair of legs: the specific differences 

 also are principally given by these organs." This relation 

 has a clear meaning on my view: I look at all the species 

 of the same genus as having as certainly descended from 

 a common progenitor, as have the two sexes of any one spe- 

 cies. Consequently, whatever part of the structure oi the 

 common progenitor, or of its early descendants, became vari- 

 ble, 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 econ- 

 omy 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 pos- 

 sessed by all the species; — that the frequent extreme varia- 

 bility of any part which is developed in a species in an extra- 

 ordinary 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 com- 

 mon 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 organisation, — are all principles closely connected to- 

 gether. 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 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 

 selection being less rigid than ordinary selection, — and to 



