108 FAUNA OF MAYFIELD'S CAVE. 



(27) Animals which, through more or less adaptation, have sought life 



in caves, usually become more modified after taking up such life 

 and are thus nicely adjusted to cave conditions. 



(28) Cave animals are modified in direct proportion to the extent to 



which they live under true cave conditions, or, to state it in the 

 reverse sense, which is just as true, cave animals are confined to 

 conditions similar to those of a cave, in so far as they have under- 

 gone modifications unfitting them for life in the open. 



(29) The rate of modification of cave animals seems hard to determine, 



in view of the fact that caves have been populated by animals in 

 all stages of adaptation to cave existence, and the age of a cave 

 furnishes no safe criterion as to the length of time the cave 

 species may have undergone modifications adapting it for cave 

 life. 



(30) It is doubtful if cave genera and species are new in the de Vries- 



ian sense, for they possess only modified or retrograde characters, 

 not new characters. 



(31) There is a certain amount of striking uniformity in the modifica- 



tions of cave animals, and in several cases convergence occurs; 

 on the other hand, there is too much diversity in the modifications 

 of cave forms for the direct effect of the uniform environment 

 readily to account for them all. 



(32) Cave genera and species have probably not arisen as mutations, 



for cave specimens of epigeal species differ from the latter 

 slightly in characters that better adapt them to cave conditions. 



(33) Most explanations of the origin of the retrograde modifications of 



cave animals involve the heredity of acquired characters. 



(34) In view of the difficulties in the way of accepting the theory of 



the hereditary transmission of acquired characters, it seems that 

 the theory of the cumulative effect of determinate variations may 

 explain the adaptations for cave life which make possible the 

 origin of cave inhabitants, and may account for their further 

 modifications after becoming cave forms. 



