SECONDARY SEXUAL CHARACTERS VARIABLE. 139 



these cases are of a very unusual nature, the relation can 

 hardly be accidental. The same number of joints in the 

 tarsi is a character common to very large groups of beetles, 

 but in the Engidee, as Westwood has remarked, the number 

 varies greatly, and the number likewise differs in the two 

 sexes of the same species. Again in the fossorial hymen- 

 optera, the neuration of the wings is a character of the 

 highest importance, because common to large groups ; but 

 in certain genera the neuration differs in the different 

 species, and likewise in the two sexes of the same species. 

 Sir J. Lubbock has recently remarked, that several minute 

 crustaceans offer excellent illustrations of this law. "In 

 Pontella, for instance, the sexual characters are afforded 

 mainly by the anterior antennae 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 species. Consequently, 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 natu- 

 ral and sexual selection, in order to fit the 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 pos- 

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

 bility 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 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 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 



