PEOTECTITE COLOEATIOJS" IN THE HOUSE-MOUSE. 469 



mice have to cope witli, it is very natural that a protectively- 

 coloured race should be gradually supplanting the dark one that 

 gave rise to it. 



Isolation, no doubt, has also been a very important factor in 

 intensifying the effects of competition ; the absence of direct 

 communication with the mainland, and the consequent impossi- 

 bility of frequent immigrations of dusky specimens from the 

 houses on the adjoining shore, have allowed Natural Selection, 

 to carry on its weeding-out of unfavourable variations without 

 disturbances of any kind. The immigrations (if any) from 

 the mainland, by swimming or by the bridge (which is made 

 of open woodwork and of considerable length), must be so very 

 exceptional, that their effects on the colony need not be con- 

 sidered. 



The alternative explanations which might be suggested do 

 not seem to me very hopeful. The objection that there may be 

 no causal relation between the factors I have enumerated and 

 the paleness of the mice, can only be established or refuted by 

 a study of the mice inhabiting similar sandy islands elsewhere. 

 Should similar races be found on such islands, the relation would 

 be at once established. That this colony may represent the 

 descendants of some foreign subspecies, introduced in a ship, is 

 unlikely when we remember the perfect intergradation between 

 our variety and the type. 



It was suggested to me that in Ireland we might have a wild 

 form of Mus musculus, as well as the introduced domestic 

 race that alone occurs in Grreat Britain. The known subspecies. 

 have recently been enumerated by Barrett-Hamilton (2). 

 Eirst, comparing the Clontarf mice with Thomas's wild race 

 from Portugal, the dimensions of the former agree with Mus 

 musculus, not with Thomas's race (1). Specimens of Mus mus- 

 culus hactrianus (Blyth), or a closely allied form, in the British 

 Museum, differ from the Clontarf mice in that their paleness 

 is due to a predominance of white tints, not of yellow. Mus 

 musculus jlavescens (Fischer) and Mus musculus sj)retus (Lataste),, 

 which according to Barrett-Hamilton probably intergrade, seem 

 to come nearest to our race. I have never seen a typical example 

 of either of these subspecies, but Southern European mice in the 

 British Museum, which are probably referable to one or other 

 of them or to something intermediate between them, are not half 

 so light as my extreme cases. In fact, the lightest mice in my 



LINK. JOUEN. — ZOOLO&T, VOL. IXTI. 34 



