GEOGRAPHIC DIVISIONS OF THE PELAGIAL 281 



1-2 m.; in the Mediterranean, on the other hand, it amounts to scarcely 

 0.5 m., in the Baltic at Jasmund 3.6 cm. on the average, at Memel 

 1 cm. The tide flat is accordingly very small in these seas, and this 

 has its effect upon the distribution of the intertidal inhabitants; for 

 example, the shore birds are much scarcer as they find much less 

 food along such a coast than along a shore whose tide flat is more 

 extensive. 



Finally, the formation of species is favored by the isolation of 

 the seas in conjunction with the peculiar conditions existing within 

 them. It is therefore to be expected that endemic varieties and species 

 should be found in these seas, which become more numerous with 

 increasing isolation. The Mediterranean and Baltic seas will be dis- 

 cussed in this connection below. In the Red Sea the number of endemic 

 animal species known to date is: 1 hexactinellid, 35 bivalves (24 lit- 

 toral and 11 from the deeper parts), 21 snails, 4 crustaceans, together 

 with 8 ptychobranchiate ascidians; 46 further investigation of ranges 

 in near-by waters may decrease these numbers. The Sulu Sea in spite 

 of its small size contains a significant number of endemic and often 

 very distinct species, e.g., 17 endemic species of macruroid fishes, 

 corresponding with representative species in the neighboring seas. 47 In 

 the Caribbean Sea a characteristic deep-water fauna has developed 

 from the descending littoral species. 



The Mediterranean Sea. — Only a few seas have been adequately 

 investigated as yet, but some, such as the Mediterranean Sea and the 

 Baltic Sea, are among the best-studied marine areas. These may there- 

 fore be further considered at this point as examples of the peculiar 

 conditions of the animal populations which have been formed by 

 reason of conditions in their special environment. 



In the Mediterranean, primarily because of the shallowness of the 

 Straits of Gibraltar and the low ratio of inflow to evaporation, both 

 salinity and bottom temperatures are higher than in similar depths of 

 the neighboring Atlantic. For these reasons the abyssal animal com- 

 munities also differ from those of the connecting ocean although other- 

 wise there is marked similarity in the two populations. 



The temperature barriers, whereby stenothermal, warmth-limited 

 animals are prevented from migrating to greater depths, disappear 

 completely. Thus the number of species of starfish which go down 

 into the lightless depths of the Mediterranean is much greater than 

 in the Atlantic. 48 Alciopidae, which as warm-water animals stay in 

 the surface layers of the ocean, are brought from great depths with 

 a sampling net. 49 Heteropod and pteropod snails, which do not ordi- 

 narily descend deeper than 300-350 m., here appear at depths of more 



