74 ANIMAL LIFE IN DESERTS 



The effects of good rain upon bird life is illustrated 

 by Ticehurst's observation that the White-cheeked 

 Bulbul {Pycnonotus I. leucotis) of Sind, which normally 

 breeds in March and April, breeds again in Sep- 

 tember in years in which much rain has fallen. 

 The fact that this bird is a denizen rather of thickets 

 and gardens than of the bare desert does not render 

 it unsuitable for inclusion in our study of the biology 

 of deserts, for the conditions of cUmate and the 

 variations between " good " and " bad " years are 

 essentially the same in the desert as in the oasis. 

 It seems that this observation and Spencer's upon 

 the small marsupials are best included here rather 

 than in the section which deals expressly with water, 

 because quite clearly the increased reproduction of 

 bird or marsupial is not to be ascribed directly to the 

 rainfall but to some much more general effect of 

 the rainfall upon vegetation, insects, etc., and so 

 indirectly upon the larger creatures. 



The present is a convenient place for considering 

 a remarkable type of small mammal which has 

 been evolved repeatedly in deserts and semi-deserts, 

 and not in any other environment : as we are 

 ignorant of the causes which have contributed to 

 the production of this type it appears best to group 

 it with other general relationships with the physical 

 environment. The mammalian type to which I 

 refer is most familiar to us in the Jerboa (Fig. 31). 

 This is a rodent about the size of a rat, provided 

 with short f ore-Umbs, which are not remarkable, and 

 excessively long hind-legs and long, narrow feet ; 

 the toes are reduced in number to two ; the tail 



