EVOLUTION 1037 



flanks, a dull bluish white under-surface, and a strongly be- 

 coloured tail, dusky above and white below. It will be under- 

 stood that these details are of little or no interest in themselves 

 and yet they too are thrilling, for they are indicative of the origin 

 of a new species. 



We come here to the true inwardness of the whole matter. Why 

 has the little island of Skomer, off the coast of Pembrokeshire, a 

 species of vole all to itself? Why is there a St. Kilda wren, found 

 nowhere else in the world? And why is there a Fair Isle field mouse, 

 different from every other mouse, though approached by its recently 

 discovered probable descendant on Foula? The answer is that 

 isolation sometimes plays an important part in species- forming, 

 and mainly by restricting the range of intercrossing, so that new 

 departures or variations tend to become racially fixed. 



The probability is that some vessel, perhaps a fishing smack 

 from the south, visiting Fair Isle many years ago, had as stowaways 

 a number, perhaps only a pair, of the common long-tailed field mice. 

 It is possible that these differed a little from the normal stock, 

 perhaps in being slightly more adventurous. Anyhow, the proba- 

 bility is that the stowaway mice landed on Fair Isle and found 

 things comfortable. In due course they had offspring, which, as 

 often happens, showed certain germinal or constitutional novelties. 

 It is possible that these new departures (variations or mutations) 

 were induced in the germ-plasm of the original settlers by some 

 idiosyncrasy in the new environment. As generation succeeded 

 generation, the new departures might be augmented by fresh 

 germinal advances in the same direction (orthogenesis). But in any 

 case, if the new departures were not fatally disadvantageous, they 

 would tend to become more and more marked by the pairing of 

 similar forms. For there were no dissimilars to cross with. So the 

 Fair Isle species arose, and so the Foula species is arising. 



SPECIES AND HABITAT 



The Pocket Gopher. — We should not speak of the earthworm, 

 for there are many different kinds; and the same is true of the 

 Pocket Gopher. It is not one but many, and they spread over the 

 whole United States west of Indiana and the lower Mississippi, and 

 more besides. Anatomically, they are not very far removed from 

 mice and rats, but they are broader in body, and marked by a big 

 pocket in the skin of each cheek, which they use for carrying food 

 home to their burrows. 



The damage they do is immense, for they devour the roots and 

 bark of fruit trees and all sorts of vegetables and cereals. They spoil 

 the meadows by throwing up hillocks of loose earth, often as 



