116 



THE AMERICAN NATURALIST 



[Vol. LI 



have seemed to tliem to prevent the random and unrestricted 

 spreading of animals over the surface of the land. The fol- 

 lowing is a list of the factors which various writers have nomi- 

 nated as affecting the distribution of the higher vertebrate ani- 

 mals. This list is complete only to the extent that my own exam- 

 ination of the literature is so. Many of the items have been 

 found in dissertations upon bird migration, which is, of course, 

 but one phase of the general subject of distribution. 

 Vegetation. 



Food supply, kind and quantity. 

 Rainfall. 



Humidity of the air (relative or absolute). 

 Wetness or dryness of the soil. 

 Barometric pressure, or altitude. 

 Atmospheric density. 

 Safety of breeding places. 

 Availability of temporary refuges. 

 Water (to land species). 

 Land (to aquatic species). 



Nature and availability of cover, or shelter from enemies. 

 Nature of the ground (coarse or fine soil, or rock). 

 Insolation, or light intensity. 

 Cloudiness. 



Temperature: in general; mean annual; of winter; of pei'iod 

 of reproduction ; of hottest part of year. 



Interspecific pressure, or competition, or race antagonism. 

 Parasitism, 



Individual, or racial, preferences. 



It is at once plain that some of the items enumerated are ex- 

 tremely complex, and that the most superficial analysis will 

 show some duplication among them. For example, the factor of 

 vegetation as influencing the distribution of different mammals 

 resolves itself principally into the elements of food-supply and 

 shelter, and, subordinately in most cases, into those of tempera- 

 ture, humidity, and nature of the soil. As some of the suggested 

 factors may really never function in any vital degree as sup- 

 posed, the total number of really critical factors is probably 

 smaller than the total of the items just listed. Time could not 

 here be taken to discuss the intrinsic nature of each elemental 

 factor, even if the writer were equipped to handle such a variety 



