396 MR. Q. E. H. BARRETT HAMILTON ON [Apr. 3, 



The fact remains, however, that iu these subspecies the young are 

 more strikingly differentiated than the adults. In all, the young 

 are duller than their respective adults, but in the East they seem 

 to take a longer time to don the brighter hues of maturity and 

 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 Mug sylvaticus must be regarded as a modern 

 acquirement, the original ancestor of the Long-tailed Field-inice 

 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, and 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 repre- 

 sented 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 distri- 

 bution. We may look on the discontinuous distribution of such 

 a form in two ways. We may regard it as evideuce 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. R. F. ScharfP as Limax 

 marginatus, var. nov. niger. This, a small dark form, was found 

 by its describer and Mr. Gr. H. Carpeuter " 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 

 mountaiu. According to Professor Simroth this form occurs also 



o 



1 There are other Eastern Nitrides (e. g. Mas confucianiis) of which the 

 young are dullr coloured and the adults red. 

 * ' Irish Naturalist,' Oct. 1899, p. 214. 



