148 DISTINCT SPECIES PRESENT 



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 vv^ith 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 varia- 

 bility 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 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 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. 



DISTI2q"CT SPECIES PRESENT AN"ALOGOUS VARIATIONS, SO 

 THAT A VARIETY OF ONE SPECIES OFTEN ASSUMES A 

 CHARACTER PROPER TO AN ALLIED SPECIES, OR RE- 

 VERTS TO SOME OF THE CHARACTERS OF AN EARLY 

 PROGENITOR. 



These propositions will be most readily understood by 

 looking to our domestic races. The most distinct breeds 

 of the pigeon, in countries widely apart, present sub-varie- 

 ties with reversed feathers on the head, and with feathers 



