No. 602] SHORTER ARTICLES AND DISCUSSIONS 



115 



FIELD TESTS OF THEORIES CONCERNING DISTRIBU- 

 TIONAL CONTROL^ 



The conditions of animal distribution and the causes of tliese 

 conditions are facts which concern intimately the problems of 

 the persistence and of the evolution of species. The present 

 writer believes that the field naturalist is in a position to con- 

 tribute in large measure toward the solution of these problems, 

 and it is the purpose of this paper to show how comparative 

 studies in the distribution of species may throw light not only 

 upon the nature of the environmental complex, but also on the 

 relative importance of its various component factors. 



Some simple facts of distribution which are of common obser- 

 vation, and which were early recorded by the systematic zoolo- 

 gist, are: (1) that each animal occupies a definite area, that is, 

 has a habitat or range, which is distinctive enough to be included 

 among the characters of the species and described along with its 

 habits and the features of its bodily structure; (2) that some 

 species (and even some of the higher systematic groups) range 

 widely, and cover great extents of country, while others are ex- 

 tremely local or restricted in their distribution; and (3) that, 

 notwithstanding considerable variation in this degree of distri- 

 butional restriction, many species (or higher groups) are found 

 nearly or entirely to coincide in range, so that sets of species, of 

 varying ranks, may be recognized distributionally, as constitut- 

 ing realms, zones, faunas, subfaunas, associations, etc. 



Perhaps the most prominent delimiting factor, and the one 

 which has been emphasized through repetition in the early sys- 

 tematic writings, is the obvious one of physical barriers — repre- 

 sented by bodies of water in the case of the terrestrial species 

 and by land in that of the aquatic. The majority of animals 

 inhabiting islands and seas are specialized in such a manner as 

 to be hemmed in by the limits of their respective habitats. In- 

 dividuals overstepping the barrier in either case are subject to 

 prompt destruction. This obvious type of distributional control 

 has always been and will remain an important one for consid- 

 eration ; but with the acquisition of detailed knowledge regard- 

 ing the distribution of animals on large continental areas, 

 naturalists have been led to propose many other factors which 



1 Contribution from the Museum of Vertebrate Zoology of the University 



