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Show 146 LAWS OF VARIATION. [0HAP. V. the s ecies, and will not be left ~o .the ~mutu.al actions. of the c~ndition of life and of a s1milar Inherited .constltu-t. It might further be expected that the species of the slaOnll. 'b' . t 1 t 1e genus would occasionally exhi It reversions o os ancestral characters. As, ~owever, we never know the exact character of the common ance~tor of .a group, we could not distinguish these two cases : If, for Instance, we did not know that the rock-pigeon was not feather-footed or turn-crowned, we could not have told, w~ether these characters in our domestic breeds were reversions or only analogous variations; but we might have inferred that the blueness was a case of reversion, from the number of the markings, which are correl~ted with the blue tint, and which it does not appear probable would all appea~ together from simple variation. More especially we might have inferred this, from the blue colour and 1narks often appearing when distinct breeds of diverse colours are crossed. Hence, though under nature it must gen~rally be left doubtful what cases are reversions to an anciently existing charact'e r and what are new but ana1 o gous vari.a - tions, yet we ought, on m~ theory, s?metimes to :fin~ the varying offspring of a spemes assum1~g. charac~ers ( mther from reversion or from analogous variation) which already occur in some other members of the same group. And this undoubtedly is the case in n.ature. . .. A considerable part of the di~culty m ::ecognising. a variable species in our systematic works, IS due ~o Its varieties 1nocking, as it were, some of the other spemes of the same genus. A considerable catalogue, also, could be given of forms intermediate between two other fo.rms, which themselves rnust be doubtfully ranked as either varieties or species ; and this shows, unless all these forms be considered as inde1>endently 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 p~rt or organ in an allied species. I have collected a long hst of ~u~h cq,~e~; but here, as before, I lie under a great disad- CJIAP. V.] LAWS OF VARIATION. 14'7 vantage in not being able to give them. I can only repeat that such cases certainly do occur, and seem to me very remarkable. ~will, however,, give one curious and complex case, not 1n~eed .as affecting an:r important character, but from occurring In several spemes of the same genus partly under domestication and partly under nature. ' It is a c~s~ apparently of reversion. The ass not rarely has very distinct tra~sverse bars on its legs, like those on the legs of the zebra : It has been asserted that these are plainest in th~ foal, and from inquiries which I have made, I believe tlus to be true. It has also been asserted that the stripe on each shoulder is sometimes double. The shoulderstripe. is certainly very variable in length and outline. A W~Ite ass,. but not an albino, has been described without e1~her spinal or shoulder stripe; and these stripes are sometimes very obscure, or actually quite lost, in darkcoloured asses. The koulan of Pallas is said to have been seen with a double shoulder-stripe. The hemionus has no shoulder-strip.e; 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, though so plainly barred like a zebra over the body, is without. bars ~n the legs ; but Dr. Gray has figured one spemmen 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, a~d of all colours ; transverse bars on the legs are not rare 1n d~ns, mouse-duns, and in bne instance in a ?hestnut : a faint shoulder-stripe may sometimes be seen 1n duns, and I have seen a trace in a bay horse. My son mad~ a careful exa~ination and sketch for me of a dun Belg1a?- cart-ho~se With a double stripe on each shoulder and With leg-st1?pes; and a man, whom I can implicitly trust, has examined for me a small dun Welch pony with three short. parallel stripes on each shoulder. · , In ~he north-west part of India the Kattywar breed of horses Is so generally striped, that, as I hear from Colonel |