194 DISTINCT SPECIES PKESENT [Chap. V, 



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 

 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 Species 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 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-varieties with reversed 

 feathers on the head, and with feathers on the feet, — 

 characters not possessed by the aboriginal rock-pigeon ; 

 these then are analogous variations in two or more 

 distinct races. The frequent presence of fourteen or 

 even sixteen tail-feathers in the pouter may be con- 

 sidered as a variation representing the normal structure 

 of another race, the fantail. I presume that no one will 

 doubt that all such analogous variations are due to the 



