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Undoubtedly there is one most important point of difference between varieties and species; namely, that the amount of difference between varieties, when compared with each other or with their parent-species, is much less than that between the species of the same genus.
Blyth and Captain Hutton, that whole flocks of these crossed geese are kept in various parts of the country; and as they are kept for profit, where neither pure parent-species exists, they must certainly be highly or perfectly fertile. With our domesticated animals, the various races when crossed together are quite fertile; yet in many cases they are descended from two or more wild species.
Varieties generally have much restricted ranges: this statement is indeed scarcely more than a truism, for if a variety were found to have a wider range than that of its supposed parent-species, their denominations ought to be reversed.
We can thus understand how it is that the females of allied species generally resemble one another so much more closely than do the males. They thus shew us approximately the primordial colouring of the parent-species of the group to which they belong.
Here it is proved that the older character dominates the younger one. In most cases of wild, and of garden-varieties, the relation between them and the parent-species rests upon comparative evidence. Often the variety is known to be younger, in other cases it may be only of local occurrence, but ordinarily the historic facts about its origin have never been known or have long since been forgotten.
The second and third heads indicate the real progressive elementary species, first those which are as strong as the parent-species, and secondly a group of weaker types, apparently not destined to be successful. Under the fourth head I shall include some inconstant forms, and under the last head those that are organically incomplete.
Spangled Hamburgs, however, offer a partial exception; for the two sexes, though not quite alike, resemble each other more closely than do the sexes of the aboriginal parent-species; yet they acquire their characteristic plumage late in life, for the chickens are distinctly pencilled.
As Fries has well remarked, little groups of species are generally clustered like satellites around other species. And what are varieties but groups of forms, unequally related to each other, and clustered round certain forms that is, round their parent-species.
Now let us apply these facts and the above two principles which latter, though not proved true, can be shown to be in some degree probable to species in a state of nature. Let us take a genus of birds, descended on my theory from some one parent-species, and of which the several new species have become modified through natural selection in accordance with their diverse habits.
Of two hundred seeds one became a blue atavist, or rather vicinist, while all others remained true to the white type. The same was observed in the white creeping thyme, or Thymus Serpyllum album, and the white self-heal, Brunella vulgaris alba, gave even so much as 28% seedlings with purple corollas out of some 400 specimens, after being cultivated in close proximity to its parent-species.
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