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Show 88 PROF. W. B. BENHAM ON THE [Feb. 7, of gradation from those which had white bellies to those which exhibited the characters of perfectly typical Mus musculus. I think, then, that we may safely conclude that Mus musculus is of at least several hundred years' standing at St. Kilda. There is one extremely interesting point which should not be forgotten in connection with these tAvo St. Kilda Mice, namely the fact that, we have here a clear opportunity of studying the effect on two distinct species of the same genus of isolation side by side on the same island. Here Ave have on a circumscribed area two species in the course of evolution, the progress of which may be easily studied from time to time. The species haA'iog now been described, we may be able in 20 or 30 years' time, by comparing specimens taken then and now, to estimate the amount of change which they will in that time have undergone. It is interesting to note, however, that so far the effect of isolation on the island is not similar in the case of the tAvo species, since apparently the Mouse which must be supposed to have been the longer time at St. Kilda is the very one which has varied in a lesser degree than that which we must regard as an introduction. For Mus hirtensis, which appears to have been on St. Kilda since that island Avas in connection with the mainland, is certainly not much more different from Mus sylvaticus than is Mus muralis from Mus musculus, yet Mus muralis can only be an introduced species of at most a few hundred years' standing. Nothing can give stronger emphasis to the fact that different species possess different powers of variability and follow a different course of evolution, so that it seems that we cannot predict Avhat Avill happen under certain circumstances to one species from our experience of what has happened to another. Every species, it would appear, has its OAVH modes of evolution and development, Avhich are peculiar to it and to it alone. EXPLANATION OF PLATE IX. Fig. 1. Mus hirtensis, p. 81. Fig. 2. Mus muralis, p. 81. 4. Notes on the Internal Anatomy of Notornis. By W. B L A X L A N D B E N H A M , D.Sc, M.A., Professor of Biology, University of Otago, Dunedin, N e w Zealand. [Received December 7, 1898.] Early in August of the present year, 1898,1 had the opportunity of examining the anatomy of that rare flightless Rail, Notornis mantelli, of which only three previous specimens had been obtained during the last 50 years, so that it has been regarded by European zoologists as probably extinct. Thus Gadow says, in Bronn's ' Thierreich ': " kiirzlich ausgestorben " (Systematic part, p. 182). The previous specimens did not reach the hands of naturalists iu a condition fit for examination, but this fourth one arrived in a |