CHAPTER V. LAWS OF VARIATION.
10. 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 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 as commonly called roots, 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, not to the
vera causa of community of descent, and a consequent tendency to vary in a
like manner, but to three separate yet closely related acts of creation.
Many similar cases of analogous variation have been observed by Naudin in
the great gourd family, and by various authors in our cereals. Similar
cases occurring with insects under natural conditions have lately been
discussed with much ability by Mr. Walsh, who has grouped them under his
law of equable variability.
With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars on
the wings, white loins, a bar at the end of the tail, with the outer
feathers externally edged near their bases with white. As all these marks
are characteristic of the parent rock-pigeon, I presume that no one will
doubt that this is a case of reversion, and not of a new yet analogous
variation appearing in the several breeds. We may, I think, confidently
come to this conclusion, because, as we have seen, these coloured marks are
eminently liable to appear in the crossed offspring of two distinct and
differently coloured breeds; and in this case there is nothing in the
external conditions of life to cause the reappearance of the slaty-blue,
with the several marks, beyond the influence of the mere act of crossing on
the laws of inheritance.