BARRIERS AND MEANS OF DISPERSAL 57 



directions, and in different organisms the capacity for dispersal is 

 also very unequal. Definite limits are set by the physical and organic 

 environment and by the constitution of the organisms themselves. The 

 possible distribution accordingly depends on the one hand upon the 

 barriers present, and on the other hand upon the means of dispersal 

 at the command of the organism in question and on its vitality and 

 adaptability. Thus definite patterns of distribution appear, and well- 

 marked highways may be found by which dispersal has taken place. 

 These highways are not common to all animals. They are conditioned 

 by and change according to the ecological valence and the means of 

 dispersal, according to the classes and orders, and even to the genera 

 and species to which the animals belong. Barriers and means of dis- 

 persal are fundamentally different for aquatic and terrestrial animals; 

 and among the aquatic animals, in turn, the relations are entirely 

 different for marine forms and for fresh-water species. 



Barriers and means of dispersal for marine animal life. — The 

 oceans form a single vast continuous mass of water, only partially 

 separated into divisions by the continents, so that these divisions have 

 no sharply defined limits. Only a few basins are rather sharply 

 delimited, such as the Mediterranean, the Baltic, and others; but 

 these, too, are connected with the oceans by straits. Barriers to dis- 

 persal are thus fewest for marine animals, and those that do exist 

 are more or less intangible. A world-wide distribution among mam- 

 mals is only found among the marine whales, such as the sperm 

 whale, the beaked whale, or the killer whale. 



Land masses inserted between the oceans naturally form insur- 

 mountable barriers to aquatic animals, even when they are narrow, 

 like the Isthmus of Panama or of Suez. Strong swimmers, and other 

 widely distributed pelagic forms, can, of course, pass around the 

 continents to the north or south unless prevented by some other 

 factor. The irregularities in the topography of the ocean floor, such 

 as ridges, deeps, and channels, are much less marked than on land, but 

 nevertheless may form effective barriers in some places for the bottom- 

 inhabiting animals although, in themselves, they afford no hindrance 

 to the spread of pelagic forms. 



Temperature, however, sets a limit to the distribution of many 

 stenothermal warmth-tolerant marine animals. The warm parts of 

 the oceans are separated from each other by cold regions at the poles. 

 The Indian and Pacific oceans are continuous in the warm zone; the 

 Atlantic is separated from them by colder waters. In consequence, 

 the uniformity of the marine life, from the East African coast through 

 the entire Indian Ocean and far into tropical Polynesia, is very 



