116 DISTINCT SPECIES PRESENT [CHAP. V. 



ance, 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 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 extreme 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 differ- 

 ence in closely allied species ; that secondary sexual and ordinary 

 specific differences are generally displayed in the same parts ot 

 the organisation, 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 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 variations in the same parts having been 

 accumulated by natural and sexual selection, and having been 

 thus adapted for secondary sexual, and for ordinary purposes. 



Distinct SjKcies present analogous Variations, so that a Variety 

 of one Species often assumes a Character proper to an allied 

 Species, or reverts to some of the Characters of an early Pro-, 

 genitor. These propositions will be most readily understood by 



