106 FAUNA OF MAYFIELD'S CAVE. 



environment suited to their morphological and physiological peculiari- 

 ties. There seems to be good evidence that caves have become colo- 

 nized by animals slightly adjusted to cave conditions, and by others 

 more or less highly modified and correspondingly well suited to cave 

 habitation. It is difficult to explain the adaptations of cave animals by 

 the action of natural selection acting upon ordinary fluctuating vari- 

 ations. If we assume the heredity of the effects of disuse and lack of 

 light there is afforded an hypothetical solution of the degeneration of 

 eyes and pigment. In view of the manifest difficulties in connection 

 with the theory of the heredity of acquired characters, it seems that 

 the equally well supported theory of determinate, cumulative variations 

 may account for the origin of cave forms and their further modifica- 

 tion after becoming cave forms. 



SUMMARY. 



(1) Conditions of temperature and moisture are quite uniform in the 



innermost parts of the cave. 



(2) A small cave presents the same diversity and all the essential con- 



ditions of a large cave. 



(3) There is little seasonal change in temperature in remote parts of 



the cave. 



(4) Air-currents through the cave are directly instrumental in effect- 



ing whatever seasonal changes in temperature the cave under- 

 goes. 



(5) The activity of cave animals is profoundly affected by changes in 



temperature, but the changes are too small to be effective in re- 

 mote parts of the cave. Near the mouth, where the changes are 

 considerable, many animals retire into cracks and crevices with 

 the lower winter temperature, while with all cave species there 

 seems a direct relation between the temperature in the cave and 

 their activity. 



(6) A small cave affords practically the whole cave fauna of a cave 



region. 



(7) Species do not often get into caves by accident. 



(8) Temporary residents are perfectly at home and come into and 



leave the cave freely. Except some mammals, they usually do 

 not go beyond twilight. 



(9) Insects hibernate in the cave in considerable numbers, but seldom 



in twilight. 



(10) Many cave inhabitants sustain a certain definite relation to the 



light, some living only in absolute darkness, others in darkness 

 and dim twilight, still others only in a certain degree of twilight. 



