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Show 396 MR. G. E. H. BARRETT-HAMILTON ON [AVT* ^> The fact remains, however, that in these subspecies the young are more strikingly differentiated than the adults. In all, the young are duller than their respective adults, but in tbe East they seem to take a longer time to don the brighter hues of maturity aud the manner of effecting tbe change is more patchy than that in vogue in the West. It may be that in regions where food is abundant and general bodily growth is rapid, the development of the genital organs cannot keep pace with that of the general size, and that with these organs the assumption of the external colours of the adult, which we know to have in many cases a most intimate connection with them, is retarded accordingly. If it be true that the various stages of progress to maturity repeat in some degree the phylogeny of an animal, then we may, perhaps, assume that the brighter red hues of the adults of several of the subspecies ' of Mus sglvaticus must be regarded as a modern acquirement, the original ancestor of the Long-tailed Field-mice having been a plain coloured, white-bellied House-mouse-like creature. Further, on the same supposition, the adult winter coat being the darker, is nearer to that of the young, aud hence to that of the ancestor, whence perhaps it might be inferred that the immediate ancestor was an animal which inhabited a damp, only moderately warm, sunless country, and is most nearly represented in its colour by the subspecies of Western Europe of the present day. The occurrence of a small dark form in such isolated localities as Lewis, Skye, Galway, and Kerry, and possibly in Portugal, seems to suggest some thoughts on our present views of distribution. W e may look on the discontinuous distribution of such a form in two ways. W e mav regard it as evidence of the survival in isolated localities of an old subspecies, once of far wider distribution ; and this is the view that would undoubtedly be adopted had we to deal in this case not with a subspecies, but with a genus or even with a very distinct species. An alternative view may, however, present itself, viz., that we may have here a case of the independent evolution of a similar form under the influence of similar conditions, which in fact one is tempted to regard as a similar reaction of the organism to the impetus of similar stimuli. To the latter view I am, I confess, myself very much inclined, and parallel cases can be found amongst other groups, as, for instance, in the case of the slug recently described by my friend Dr. B. F. Scharff2 as Limax marginatus, var. nov. niger. This, a small dark form, was found by its describer and Mr. G. H . Carpenter " suddenly " appearing at a height of 2800 feet on Carrantuohill, in Co. Kerry, Ireland, and was certainly quite absent from the lower slopes of the mountain. According to Professor Simroth this form occurs also 1 There are other Eastern Murida (e. g. Mus confucianus) of which the young are dullv coloured and the adults red. * ' Irish Naturalist,' Oct. 1899, p. 214. |