

ENVIRONMENTAL SELECTION 29 



Adaptive selection. — The necessity for definite adaptations acts 

 upon the fauna like a sieve of definite mesh, allowing only more or 

 less similar forms to pass. Thus arise the common characters of ani- 

 mals living under similar conditions, which are the more striking the 

 more closely the environmental conditions approach to limiting val- 

 ues. Accordingly, characters common to a fauna are most notable in 

 deserts, in the polar regions, and in inhabitants of temporary pools. 

 Such common traits are difficult to discover in animals of large bodies 

 of water or in the terrestrial animals of temperate zones. In the trop- 

 ical seas, where optimum conditions reign, such adaptations to the 

 environment are less and less important, and are restricted to special 

 habitat conditions, like the floating arrangements of plankton or the 

 adhesive apparatus of the inhabitants of surf-beaten rocks. 



Another interesting mutual relation may be explained by means 

 of these factors, which condition the wealth of form in a given environ- 

 ment. Where the number of morphologic types is greater the struggle 

 for existence between the different species of animals will be much 

 more violent than in areas where the fauna is poorer in representatives 

 of diverse structure. In the former case (e.g., in the tropics) there is 

 much more severe competition for the same goal, and the opportunities 

 to get the better of a competitor are much more numerous. Even 

 predatory animals are compelled to protect themselves against enemies 

 of many kinds, and he who escapes Charybdis falls into the jaws of 

 Scylla. Where the struggle for existence between animal competitors 

 is reduced as a result of the reduction in number of types, as in fresh 

 water, in the steppes, in deserts, or in the polar regions, then, con- 

 comitantly, the struggle with the physical forces of nature becomes 

 more severe. 



Many animals are able to maintain themselves in physically un- 

 favorable environments after they have given way elsewhere to more 

 modern competitors. Thus the last representatives of the lampreys and 

 lungfishes persist in fresh water. So do the ganoids, some of which are 

 completely restricted to fresh waters, while others, like the sturgeons, 

 repair to them to spawn. Among the bony fishes, the more primitive 

 soft-rayed forms, in competition with the spiny-rayed Acanthopter- 

 ygii, have maintained themselves everywhere under more adverse en- 

 vironmental conditions, as in the North, in the oceanic depths, and in 

 fresh water, where four-fifths of the physostome species are found. 



Although the persistence of some species under increasingly adverse 

 conditions may produce degeneration, as in Nereis diversicolor in 

 saline springs, or in some protozoans in hot springs, these cases are 

 exceptional. In general, adaptation to an environment enables an 



