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Updated: May 17, 2025
These organs in the male often differ in closely-allied species, and afford excellent specific characters. But their importance, from a functional point of view, as Mr. R. MacLachlan has remarked to me, has probably been overrated.
I may here mention that Judge Caton has in his park three races of the Virginian deer, which differ slightly in colour, but the differences are almost exclusively confined to the blue winter or breeding-coat; so that this case may be compared with those given in a previous chapter of closely-allied or representative species of birds, which differ from each other only in their breeding plumage.
But when we find that it is not universal; that in two closely-allied species one will possess it and the other not; and that it is common in such widely-separated orders as gallinaceous and passerine birds, in pigeons, ducks, and waders, it becomes plain that it is not assignable to community of descent, but has originated independently all over the globe, in a vast number of species.
Wallace's formula: "Every species has come into existence coincident both in time and space with preexisting closely-allied species." Edward Forbes, who may be called the originator of this whole line of inquiry, long ago expressed a similar view.
As he extends the range of his observations, he will meet with more cases of difficulty; for he will encounter a greater number of closely-allied forms.
The conclusion that the horns have been acquired as ornaments is that which best agrees with the fact of their having been so immensely, yet not fixedly, developed, as shewn by their extreme variability in the same species, and by their extreme diversity in closely-allied species.
And wherever many closely-allied species occur, there will be found many forms which some naturalists rank as distinct species, and some as varieties; these doubtful forms showing us the steps in the process of modification.
"During the voyage of the 'Beagle' I had been deeply impressed by discovering in the Pampean formation great fossil animals covered with armour like that on the existing armadillos; secondly, by the manner in which closely-allied animals replace one another in proceeding southwards over the continent; and, thirdly, by the South American character of most of the productions of the Galapagos Archipelago, and, more especially, by the manner in which they differ slightly on each island of the group; some of the islands appearing to be very ancient in a geological sense.
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