236 THE VARIATION OF ANIMALS IN NATURE 



It seems to us that the first thing to discover is how far, 

 in specific instances, particular homochromatic species do 

 match their background. Naturally we cannot discuss more 

 than a few instances, and it may be felt that we have 

 exercised an arbitrary selection. The cases chosen are ones 

 which have been claimed as demonstrating a correlation 

 between colour and habitat on the ground of accurate field 

 work. For this to be convincing in proving the selective value 

 of the colour it is not enough, of course, to find (e.g.) a few 

 pale-coloured rodents on a sand-spit. We ought to be able to 

 show that the resemblance occurs over at least half the range 

 of the race or species. We have introduced one ' difficult ' 

 or negative case (Peromyscus) which clearly demonstrates how 

 difficult it is to get agreement and exact evidence on a subject 

 like this. 



Dark coat-colour of Rodents on lava fields. 



Dice (1929) and Benson (1932) have described dark forms 

 of rodents from the dark lava fields of Central America and 

 Mexico. Benson (I.e. p. 336) is very guarded as to the exact 

 correlation of soil and coat-colour, because ' there are other 

 dark races of rock-squirrels in the south-west concerning which 

 there is little information available as to whether any relation 

 exists between their dark colour and their environments, and, 

 furthermore, one of these races (Citellus grammurus couchii) . . . 

 exhibits dichromatism. It may be of significance, however, 

 that the range of each of these dark races includes areas of 

 dark-coloured rock.' The Guadalupe Mountains, which are of 

 a paler sedimentary rock, are inhabited by the (paler) typical 

 C. grammurus grammurus. 



Sumner (1921, p. 75), who made an intensive study of 

 Peromyscus on lava fields, could find no evidence of any higher 

 incidence of dark types on the lava than on the adjacent brown 

 loamy soil. Sumner's tables of the incidence of the various 

 colours on divers backgrounds are very conclusive. 



Pale race of Peromyscus on white sand-spit. 



Sumner (1928) found that P. polionotus leucocephalus living 

 on a white coral-sand island (Santa Rosa) were lighter than 

 the race (albifrons) inhabiting the darker soil of the adjacent 

 mainland. This interesting case was re-examined by him 



