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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。
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； a white rump； 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。
No doubt it is a very surprising fact that characters should reappear after having been lost for many； perhaps for hundreds of generations。 But when a breed has been crossed only once by some other breed； the offspring occasionally show a tendency to revert in character to the foreign breed for many generations some say； for a dozen or even a score of generations。 After twelve generations； the proportion of blood； to use a common expression； of any one ancestor； is only 1 in 2048； and yet； as we see； it is generally believed that a tendency to reversion is retained by this very small proportion of foreign blood。 In a breed which has not been crossed； but in which both parents have lost some character which their progenitor possessed； the tendency； whether strong or weak； to reproduce the lost character might be； as was formerly remarked； for all that we can see to the contrary； transmitted for almost any number of generations。 When a character which has been lost in a breed； reappears after a great number of generations； the most probable hypothesis is； not that the offspring suddenly takes after an ancestor some hundred generations distant； but that in each successive generation there has been a tendency to reproduce the character in question； which at last； under unknown favourable conditions； gains an ascendancy。 For instance； it is probable that in each generation of the barb…pigeon； which produces most rarely a blue and black…barred bird； there has been a tendency in each generation in the plumage to assume this colour。 This view is hypothetical； but could be supported by some facts； and I can see no more abstract improbability in a tendency to produce any character being inherited for an endless number of generations； than in quite useless or rudimentary organs being； as we all know them to be； thus inherited。 Indeed； we may sometimes observe a mere tendency to produce a rudiment inherited： for instance； in the common snapdragon （Antirrhinum） a rudiment of a fifth stamen so often appears； that this plant must have an inherited tendency to produce it。
As all the species of the same genus are supposed； on my theory； to have descended from a common parent； it might be expected that they would occasionally vary in an analogous manner； so that a variety of one species would resemble in some of its characters another species； this other species being on my view only a well…marked and permanent variety。 But characters thus gained would probably be of an unimportant nature； for the presence of all important characters will be governed by natural selection； in accordance with the diverse habits of the species； and will not be left to the mutual action of the conditions of life and of a similar inherited constitution。 It might further be expected that the species of the same genus would occasionally exhibit reversions to lost ancestral characters。 As； however； we never know the exact character of the common ancestor 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； whether 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 correlated with the blue tint； and which it does not appear probable would all appear together from simple 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 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； come of the other species of the same genus。 A considerable catalogue； also； could be given of forms intermediate between two other forms； which themselves must be doubtfully ranked as either varieties or 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 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 same 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 of a 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 shoulder…stripe； but traces of it； as stated by Mr Blyth and others； occasionally appear： and