EVOLUTION 1035 



disputable question. But Kammerer's work brings out very con- 

 vincingly the importance of spatial isolation in fostering new 

 strains. He has furnished incontrovertible evidence of the importance 

 of isolation as a factor in Organic Evolution. 



The Foula Mouse. — Foula is a little rocky island out in the 

 Atlantic, about 16 miles west of the nearest point of the mainland 

 of Shetland. As everyone doesn't know that it is the Ultima Thule 

 of Tacitus, we mention this in passing. It is about three miles long 

 and half as broad, with high cliffs where guillemots and razorbills 

 and other sea-fowl nest in earty summer and the storms rage in 

 winter. On the rough ground from the foot of the hills up to 1,000 

 feet above sea-level the famous "Hill Mice" have their home. The 

 mountains have brought forth a mouse, a mouse that is all their 

 own. 



If one is to be accurate, one must ask whether a long-tailed little 

 creature is a mouse or a vole ; or whether a mouse is a Field Mouse 

 (Apodemus), or a Harvest Mouse (Micromys), or a House Mouse 

 (Mus). This is not a question of species, but of genera represented 

 in Britain. Thus, apart from small voles (Microtus), such as the 

 Field Vole, and big voles (Arvicola), such as the "water rat", which 

 isn't a rat, there are three British genera of mice, namely, as we 

 have just said, Apodemus, Micromys, and Mus. A small vole can 

 be at once distinguished from a mouse of the same size by the 

 broader head, "the blunt muzzle, the smaller eyes, the appressed 

 ears, and the short, hairy tail. When is a mouse not a mouse? 

 Answer: When it is a vole. When is a rat not a rat? Answer: When 

 it is a water-vole. 



So far all is easy; the difficulties begin when we come to species, 

 by which we mean true-breeding groups of similar individuals 

 which are readily fertile with one another, but not readily with 

 others, and which differ from the nearest group in characters more 

 marked than those that distinguish the members (brothers and 

 sisters) of one family, and important enough to deserve a name to 

 themselves. This is a useful working definition of a species. But the 

 species of mice, there's the rub ! It is here that we need the help of 

 the people who really care about precision and who have a head 

 for it. 



Revenons a nos moutons, that is to Foula mice. In the first place 

 they are Field Mice in the genus Apodemus, along with the common 

 Long-tailed Field Mouse {Apodemus sylvaticus), which occurs all 

 over Britain, and is probably the commonest mammal in Europe. 

 Another well-known species is the Hebridean Field Mouse (A, 

 hehridensis) , of which there are four or five geographical varieties. 

 A third species is the St. Kilda Field Mouse {A. hirtensis), 

 found on the islands of Hirta, Soay, and Dun. A fourth species is 



