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Show 162 LAWS OF VARIATION. CHAP. V. cases: if, for instance, we did not know that the rockpigeon was not feather-footed or turn-cro~ned, we cou~d not have told whether these characters In our domestic breeds were :eversions or only analogous variations; but we might have inferred that the bluenes~ was a ?ase of reversion from the number of the markings, which are correlated with the blue tint, and which it does not appear probable would all appear. together .from simp!e variation. More especially we might have Inferred this, from the blue colour and marks so often appearing when distinct breeds of diverse colours are crossed. Hence, though under nature it must generally be left doubtful, what cases are reversions to an anciently existing character, and what are new but analogous variations, yet we ought, on my theory, sometimes to find the varying offspring of a species assuming characters (either from reversion or from analogous variation) which already occur in some other members of the same group. And this undoubtedly is the case in nature. A considerable part of the difficulty in recognising a variable species in our systematic works, is due to its varieties mocking, as it were, some of the other species of the same genus. A considerable catalogue, also, could be given of forms intermediate between two other ~orms, which themselves must be doubtfully ranked as either varieties or species ; and this shows, unless all these forms be considered as independently created species, that the one in varying has assumed some of the characters of the other, so as to produce the intermediate form. But the best evidence is afforded by parts or organs of an important and uniform nature occasionally varying so as to acquire, in some degree, the character of the same part or organ in an allied species. I have collected a long list of such cases; but CHAP. v. LAWS OF VARIATION. loo here, as before, I lie under a great disadvantage in not being able to give them. I can only repeat that such cases certainly do occur, and seem to me very remarkable. I will, however, give one curious and complex case, not indeed as affecting any important character, but from occurring in several species of the sarn.e genus, partly under domestication and partly under nature. It is a case apparently of reversion. The ass not rarely has very distinct transverse bars on its legs, like those on the legs of the zebra: it has been asserted that these are plainest in the foal, and from inquiries which I have made, I believe this to be true. It has also been asserted that the stripe on each shoulder is sometimes double. The shoulder-stripe is certainly very variable in length and outline. A white ass, but not an albino, has been described without either spinal or shoulder stripe; and these stripes are sometimes very obscure, or actually quite lost, in dark-coloured asses. The koulan of Pallas is said to have been seen with a double shoulderstripe. The hemionus has no shoulder-stripe; but traces of It, as stated by Mr. Blyth and others, occasionally appear : and I have been informed by Colonel Poole that the foals of this species are generally striped on the legs, and faintly on the shoulder. The quagga, t~ough so plainly barred like a zebra over the body, is Without bars on the legs ; but Dr. Gray has figured one specimen with very distinct zebra-like bars on the hocks. With respect to the horse, I have collected cases in England of the spinal stripe in horses of the most distinct breeds, and of all colours ; transverse bars on the legs are not rare in duns, mouse-duns, and in one instan~e in a chestnut : a faint shoulder-stripe may sometimes be seen in duns, and I have seen a trace In a |