THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 39 



tion of species is far less easy and definite than in recent biology, the appear- 

 ance of new forms whose genesis and relationships are obscure must not be 

 attributed solely to changes of the inorganic world. 



(i) Control of Distribution. 



To aquatic invertebrates the presence of land would seem an insur- 

 mountable barrier to all normal expansion, but it must be remembered that 

 some of the Crustacea usually aquatic have progressed so far toward a 

 terrestrial life that they endure a surprising lack of water. The common 

 crayfish {Astacus flimatilis) has gone so far in this direction that it lives 

 in very arid regions. The author has found active specimens of crayfish 

 in little rills formed by recent rains upon the driest part of the plains, and 

 found one vigorous specimen living among some damp rocks where the 

 merest trickle of water preserved moisture. It is very probable that this 

 process has been repeated many times in the past. 



The majority of invertebrates found fossil are shallow-water forms, 

 and to these a deep sea is as impassable a barrier as dry land. Shallow 

 seas were probably far more common and widespread in the Paleozoic than 

 in later times and the presence of similar members of shallow-water groups 

 in remote localities is amply sufficient for the assumption of a shallow-water 

 connection between the two places. 



The term "barrier," however, involves a most complex conception. 

 The presence of land and deep water are the simplest types of barriers to 

 migration. Localized conditions of the water may form impassable barriers. 

 Currents which have so much to do with the distribution of free-swimming 

 forms and of larvae would prevent the same forms from crossing them in 

 anything like a direct path. Warm and cold currents would be as efhcient 

 barriers as equally marked extremes of temperature upon the land. 



Alexander Agassiz reported an area almost devoid of life in the deep 

 ocean waters off the west coast of South America. What the cause of this 

 is we do not know, but for some reason the life has been barred off. 



The waters of the Atlantic Ocean off the west coast of northern Africa 

 are exceptionally saline, due to excessive evaporation. We can not doubt 

 that in the nice adjustment of life this is an efficient barrier to some forms 

 of life. 



For land invertebrates and vertebrates the barriers would be of the 

 same general kind. It is essential to recognize that it is not only the 

 physiographic features which must be considered. Mountains, lakes, rivers, 

 climate, deserts, etc., are important and effectual, but a stretch of grassland 

 or a deep forest is as effectual to forms accustomed to an opposite type of 

 habitat. Absence of food-supply is equally efficient, and animals may be 

 restricted by a physiographic or hydrographic barrier which would be 

 utterly inefficient in itself through its effect upon the vegetation. 



