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
variation in two so-called distinct species; and to these a third may
be added, namely, the common turnip. According to the ordinary view of
each species having been independently created, we should have to
attribute this similarity in the enlarged stems of these three plants,
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