LAWS OF VARIATION 127 



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 com- 

 parison with the same part in its congeners ; and the slight degree 

 of variability in a part, however extraordinarily it may be devel- 

 oped, if it be common to a whole group of species; that the great 

 variability of secondary sexual characters and their difference in 

 closely allied species; that secondary sexual and ordinary specific 

 differences are generally displayed in the same parts of the or- 

 ganization, — 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 in- 

 herited 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 nat- 

 ural and sexual selection, and having been thus adapted for sec- 

 ondary 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 feath- 

 ers 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 

 considered 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 several races of the 

 pigeon having inherited from a common parent the same constitu- 

 tion and tendency to variation, when acted on by similar un- 

 known influences. In the vegetable kingdom we have a case of 



