140 ANNALS OF SCOTTISH NATURAL HISTORY 



longer in a state of uncertain evolution. On the North Bull sand- 

 hills, indeed, Mr. Jameson found not only Mice which had pro- 

 gressed for a considerable distance along the path of their new 

 development, but also Mice which showed every kind 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 two 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 we have, on a circumscribed space, two species in 

 the course of evolution, the progress of which may be easily studied 

 from time to time. The species having now been described, 

 we can in twenty or thirty years' time, by comparing specimens 

 taken then and now, 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 two 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 one which we must 

 regard as an introduction. For Mus htrtensis, which appears to 

 have been on St. Kilda since that island was 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 what will happen 

 under certain circumstances to one species from our experience of 

 what has happened to another. Every species, it would appear, has 

 its own rules for evolution and development, which must be applied 

 to it and to it alone. 



REPORT ON THE MOVEMENTS AND OCCUR- 

 RENCE OF BIRDS IN SCOTLAND DURING 



1898. 



By T. G. LAIDLAW, 

 Member of the British Ornithologists' Union. 



THE Schedules returned for 1898 number fifty-three, an 

 increase of seven over those received for the preceding year. 

 The Lighthouses and Coast Stations furnish twenty-three 



