ANIMALS— PHYSICAL ENVIRONMENT 79 



so closely resembles it in habits and shape, is very- 

 rare, possibly because it is an insect-eater (page 83). 

 Every one of these small Jerboa-like creatm*es 

 exhibits the coloration typical of so many desert 

 animals (page 141), the sandy, unicolorous upper 

 surface, and white belly. 



For the following tables I am indebted to the 

 kindness of Mr. O. Thomas, F.R.S., of the British 

 Museum (Natural History). The first makes clear 

 the relationships of the forms of which I have 

 been speaking ; it shows that animals outwardly 

 resembling the Jerboa have been evolved on a 

 number of different occasions, apparently as a 

 response to the conditions of life in a desert, and 

 that in each continent the Jerboa-like animal has 

 been produced from a different family or sub- 

 family. 



The second table shows at a glance the number 

 of toes present in these Jerboa-Hke animals, and 

 emphasizes the fact that a reduction in the number 

 of toes has repeatedly taken place. Five is of 



