No. 567] SHORTER ARTICLES AND CORRESPONDEXCE 179 



noteworthy fluctuating variability, due to differences in ex- 

 pression. 



Mus musculus, then, is very conspicuously variable in color; 

 yet Miller's book records only one subspecies, that of the Medi- 

 terranean region and the Azores, which is less dusky and more 

 yellowish, with the under parts buffy grayish. It possibly agrees 

 with Little's "dilute black agouti" variety. On the other hand, 

 M. musculus has a recognized subspecies in Mexico, where it 

 must have developed since the species was introduced by man. 

 The mice of St. Kilda and the Faroe Islands, although given as 

 distinct species, are derivatives of Mus musculus, differing in 

 other points than color. In connection with subspecific differ- 

 ences in size, Sumner's experiments with different temperatures 

 should be noted, since they prove that differences of temperature 

 might lead to readily measurable differences in dimensions, 

 wholly unconnected with losses of determiners or new zygotic 

 combinations. Whether or not diverse conditions of this sort 

 would ultimately affect the germ plasm, their effects would be 

 patent long before and quite independently of any such modifi- 

 cation. On the whole, the poverty of Mus musculus in subspecies 

 would suggest that the variations observed by breeders are not, 

 as a rule, the stuff that new subspecies are made of. Acrninst this 

 argument may well be adduced the fact that M. musculus is an 

 urban animal, constantly traveling alwut, so that incipient races 

 do not remain isolated. Here the closely related rats, Epimys, 

 are worth considering. For Europe Miller can only recognize 

 the Norway, Black and Alexandrian rate, all widespread, prac- 

 tically cosmopolitan. Yet in the Malay Archipelago, where 

 Epimys is distributed over myriads of islands, large and small, 

 the species are innumerable. One can almost take a map and 

 indicate where new species of Epimys are to be found, namely, on 

 those islands still unexplored. Years ago, when the writer was 

 actively engaged in studying the British Mollusca and Lepi- 

 ddptera, the question of endemic forms was constantly in mind; 

 but in those days we failed to discriminate properly between the 

 different classes of ' ' varieties. ' ' We made the mistake of looking 

 for well-marked "sports" or aberrations, rather than for con- 

 stant but only slightly distinguished local races. There was a 

 Practical reason for this, in the fact that by searching the litera- 

 ture we could ascertain whether a well-marked variation had 

 been reported from the continent; whereas the determination of 

 subspecific types analogous to those described by Miller among 



