CONCLUSIONS ON CAVE ENVIRONMENT. 21 



CONCLUSIONS. 



(i) The possible physical environment of animals is composed of units, each 

 of which is distinguished by a combination of conditions peculiar to it. 



(2) A unit may embrace one continuous area. 



(3) A unit may have extended in the past over a continuous area, but may 

 now be broken up into separate, though similar, parts between which the migra- 

 tion of animals is not possible. 



(4) A unit may always have existed of separate and distinct parts (units of a 

 smaller order) which together form a discontinuous unit. 



(5) An animal distributed over a continuous, or parts of a formerly continuous, 

 unit may have arisen at a single center of dispersal. 



(6) An animal distributed over a discontinuous unit must have had separate 

 places of origin or have originated at a time when the parts of the unit were con- 

 tinuous. 



(7) Each cave consists of a twilight section, a fluctuating temperature section, 

 and the cave par excellence. 



(8) The environment in the third section is chiefly characterized by (a) the 

 absence of light ; (b) the constancy of meteorological conditions between seasons ; 

 (c) the absence of food except such as is imported. 



(9) All classes of vertebrates, except birds, have blind members. 



(10) Some cave animals (aquatic) have developed pari passu with the devel- 

 opment of an underground stream and are among the few inhabitants remaining 

 to the stream of its inhabitants during its epigean period. 



(11) Some cave animals (non-aquatic) have gradually colonized caves after 

 their formation. 



(12) Some cave animals became elsewhere adjusted to live in the dark and 

 later migrated into caves. 



(13) Accident had little or nothing to do with the colonization of caves. 



(14) Some widely distributed cave species have independently arisen in dif- 

 ferent places from a widely distributed epigean species. 



(15) Directly or indirectly all of the food supply of a cave must be imported. 



(16) Smaller caves have a relatively richer fauna, because the food supply is 

 more abundant. 



(17) Older caves have a more varied and richer fauna. 



(18) Cave animals tend to converge in their evolution; epigean animals, to 

 diverge. 



