232 THE VARIATION OF ANIMALS IN NATURE 



of the body in insects living at high altitudes, silt- and mud- 

 adaptations of estuarine invertebrata, and the like. The 

 evidence as to the origin of these modifications is so meagre 

 that it is useless to discuss them. We have, however, included 

 a short discussion on two problems which do not seem to us 

 capable of solution but are too important to dismiss summarily. 



It must be understood in the following discussion that the 

 difficult question as to the origin of habits and the relation of 

 the latter to differences of structure between species is momen- 

 tarily left out of account. We are now concerned with dis- 

 cussing to what extent there is a correlation between specific 

 differences and habitudinal ones. The question as to which 

 arose first is discussed on p. 301. 



We propose to deal with this evidence under the following 

 heads : 



A. Indirect evidence for the occurrence of Natural 



Selection. 



(a) Standard cases. 



(1) Protective resemblance and warning 



coloration. 



(2) Mimicry. 



(b) Less intensively studied cases. 



(1) Adaptation of torrent-living animals. 



(2) The colour of cuckoo's eggs. 



(3) The deep-sea fauna. 



(4) Cave animals. 



B. Difficulties raised by the Natural Selection theory. 



(1) Specific differences in colour and structure. 



(2) The problem of secondary sexual charac- 



ters. 



(3) The origin of habits. 



(4) Complex organs and ' co-adaptations.' 



A. Indirect evidence for the occurrence of Natural Selection. 



(a) Standard Cases. (1) Protective resemblance and warn- 

 ing coloration. 



These phenomena are particular aspects of the general 

 question of protection against predators, which includes such 

 devices as autotomy, menacing postures, ' shamming dead,' 

 and the development of spines and armour. We select them 



