CAVES A XL) THE CAVE FAUNA. 



CAVES IN THEIR RELATIONS TO THE REST OF THE UNIVERSE. 



The environment favorable to animal life is limited to a thin layer of water, 

 earth, and air. From its deepest to its most elevated point this layer does not 

 much exceed 10 miles ' in thickness. At no particular point does it exceed much 

 more than half this thickness ; and usually the layer is but a few feet thick. About 

 half the total thickness is below sea-level and the other half above it. The places 

 where the ocean has a depth of 5 miles are few, but in these places the greatest 

 depth of possible environment is found. The favorableness of the environment 

 diminishes rapidly with the depth. The depth of the possible environment at any 

 point on land above the surface is very limited, and beneath the surface it depends 

 on conditions ; solid rocks may limit it to the surface and soil may permit mam- 

 mals, and especially insects, to burrow several feet beneath the surface. Under- 

 ground watercourses, which are caves in the formation, may enable animals to live 

 several hundred feet beneath the surface of the ground. The animals thrown out 

 by artesian wells attest this. Typhlomolge is occasionally thrown out of the 

 artesian well 190 feet deep at San Marcos, Texas. The plant environment stops 

 at the surface of the ground ; 2 animal life diminishes rapidly within a few feet of 

 the surface unless trees cover the ground. Animal environment definitely stops 

 at the tops of trees, though the air above them may be temporarily visited. 



While the depth of the environment at any point is only a few feet on land, 

 because the surface of the land itself rises to a few miles above sea-level, the total 

 depth of the environment above sea-level is considerable. The fauna rapidly 

 diminishes in either direction from sea-level, and were it not that the extreme limits 

 of the environment, above and below, furnish rare, sometimes peculiarly adapted 

 forms, sometimes relicts, the numbers of individuals and types found would not 

 repay the exploration of the ocean depths and mountain heights. 



Since the environment varies within the limits of the possible existence of living 

 matter, from the extreme of wetness and dryness, of heat and cold, of depth and 

 height, of light and dark, etc., we may divide the environment into many distinct 

 units within which the conditions are similar or alike. It is profitable at present to 

 call attention only to discontinuous and continuous units of environment. Similar 

 or identical conditions may stretch uninterruptedly in one or more directions indefi- 

 nitely, permitting the free movement of its inhabitants from one part to another. 

 The continuous unit of environment of greatest extent is furnished by the ocean at 

 considerable depths. Light and temperature conditions and seasonal fluctuations 

 are reduced to the minimum and are nearly uniform under the whole surface of 

 the ocean, furnishing an ideal of the type of the continuous environment. This 

 particular environment is continuous not only as to space, but also as to time. 



The surface of the ocean forms an equally continuous area, but because tem- 

 perature and light conditions differ greatly in different parts of the globe we must 

 here deal not with a single but with several distinct units of environment, each large 

 in extent. If we assume the conditions in the north polar sea to be identical with 



1 Highest mountain, deepest ocean. - Some fungi are found in caves. 



3 



