CHAPTEK XXI 



Geographical distribution 1 Areas of distribution Barriers to migration 

 Means of dispersal Changes in the physical conditions of the earth's 

 surface The evidence afforded by the study of geographical distribution 

 with regard to the theory of organic evolution. 



IT is hardly necessary to remind the reader that each species 

 of plant or animal, in a state of nature, is more or less sharply 

 restricted to a certain portion of the earth's surface, the entire 

 region over which it may be found, whether sea or land, being 

 termed its area of distribution. Such areas of specific distribu- 

 tion are nearly always continuous, without any considerable gaps 

 or intervals from which the species is entirely absent. This does 

 not, of course, mean that the species necessarily occurs in all 

 parts of its area of distribution at once, but that it is free to 

 range over the whole of it and may accordingly be found in any 

 suitable part of it at any time. It is necessary to introduce some 

 such qualifying word as " suitable " in this connection, because 

 each species is not only restricted in its range to a more or less 

 well-defined geographical area, but can only live continuously in 

 certain portions of that area, to the special conditions of which it 

 is structurally and physiologically adapted and which constitute 

 its habitat. Thus, for example, a fresh-water snail may perhaps 

 range over an entire continent, but it would be useless to look for 

 it except in fresh water. Individuals of a species may pass with 

 more or less freedom, according to the nature of the case, from 

 one habitat to another within the area of distribution, but it is 

 only on rare and exceptional occasions that they are able to 

 transgress the boundaries of the area itself. 



True discontinuity in areas of specific distribution, as distin- 

 guished from mere discontinuity of habitats, is extremely rare. 

 We have a good example of it, however, in the case of the marsh 



1 The reader is referred to Dr. Wallace's classical volumes on " Island Life " and 

 the " Geographical Distribution of Animals," and to Professor Heilprin's work on 

 the " Distribution of Animals" in the International Scientific Scries (Vol. LVIII, 

 1887), for further information on this subject. 



