24 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



darkened coloration in many forms; the absence of light for cave 

 dwellers results in a fauna of creatures without eyes, commonly with- 

 out pigment, and with a resulting pale coloration. A disadvantageous 

 environment forces common traits upon the inhabitants, and the rela- 

 tion between the type of habitat and the appearance of its fauna be- 

 comes more obvious. 



Environmental selection of aquatic life. — Changes in the marine 

 fauna are principally associated with the decrease in temperature 

 which takes place with approach toward the poles and with the fac- 

 tors accompanying an increase in depth. In both cases the resultant 

 reduction in variety of animal life is notable. In contrast with the 

 limitless variation of the plankton of warm waters, the arctic plankton 

 exhibits a certain constancy of character. The cold water seems un- 

 favorable to the formation of varieties. 1 Whereas ninety species of reef 

 corals occur in the Hawaiian Islands, 2 only ten species of Madre- 

 poraria are found in the Bermudas, the northern limit of their range. 

 The families of ascidians all attain their greatest number of species 

 in the tropics; 3 the distribution of their 109 genera and of the species 

 is shown in the following table: 



Genera Species 



Arctic 34 of which 2 (5.9%) endemic 103 of which 61 (59.2%) endemic 



Subarctic. ... 60 " " 10(16.6%) " 432" " 352(81.4%) 



Tropic 71" " 21(29.6%) " 635" " 563(88.6%) 



Subantarctic. 49 " " 7 (14.3%) " 219 " " 173 (79.0%) 

 Antarctic... 22 " " 3(13.6%) " 42" " 36(86.0%) 



The small number of endemic genera in the Arctic and Antarctic, 

 with a large number of endemic species, attests the effectiveness of 

 the selective process. The number of types which can adapt them- 

 selves to the new conditions is relatively small. The Atlantic copepods 

 are distributed as follows: 4 



Between Temperature Limits of Number of Species 



27.5-20.3 34 



19.6-11.4 19 



11.1- 0. 8 



Among the marine fishes which live in shallow coastal waters, only a 

 few families are absent in the tropics, while a great number of fam- 

 ilies are confined to the tropics or subtropics, and a still greater 

 number have the majority of their representatives there. Seven hun- 

 dred and eighty species of fishes are reported from the coasts of 

 Amboyna in the Moluccas, almost as many as are present in all the 

 seas and rivers of Europe together. 



