THE COLOURS OF DESERT ANIMALS 169 



their movements : to these forms the theory of 

 protective coloration may rightly be applied. The 

 point upon which I wish to insist is that the theory 

 only covers a small portion of the facts ; we must 

 look for some other cause, and suppose that after 

 the prevalent buff or sandy colours had become 

 estabhshed, certain of the creatures used them as 

 a protection from their enemies : this is equivalent 

 to saying that the protection afforded is quite a 

 secondary matter and in no way causal. 



If we are compelled to disavow our faith in pro- 

 tective coloration as the cause of this remarkable 

 general depigmentation, to what are we to turn for 

 an explanation ? The cause is one which affects 

 animals of every type, in every desert ; it does not 

 discriminate between prey and captor, between the 

 creeping and flying animals, between the diurnal, 

 the nocturnal, and the subterranean. It is so 

 universal in its appHcation that it is probably 

 physical. No biological agency, such as the struggle 

 with rivals for food or water, the avoiding of enemies, 

 or seeking a mate, can cover so wide a range of 

 animals ; heat alone it is not, for heat in other 

 parts of the world does not produce these uniform 

 pale forms ; low or fluctuating relative humidity 

 it can hardly be, for the animals which live below 

 stones and in burrows by day, and emerge at night, 

 are not exposed either to very low or very fluctu- 

 ating humidity. I am unable to suggest any con- 

 dition, or combination of conditions, which can 

 make itseK felt so widely, but I feel that the 

 explanation will eventually be found in studying 



