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 the species possess in common;—­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 not great 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 the great amount of difference in these same characters between 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 having descended from 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 thus adapted for secondary sexual, and for ordinary specific purposes.

Distinct species present analogous variations; and A variety of one species often assumes some of the characters of 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 pigeons, in countries most widely apart, present sub-varieties with reversed feathers on the head and 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 constitution and tendency to variation, when acted on by similar unknown influences.  In the vegetable kingdom we have a case of analogous variation, in the enlarged stems, or roots as commonly called, of the Swedish turnip and Ruta baga, plants which several botanists rank as varieties produced by cultivation from a common parent:  if this be not so, the case will then be one of analogous