No. 602] SHORTER ARTICLES AND DISCUSSIONS 



127 



Summary 



In this paper I have enumerated various factors thought to 

 be concerned with the control of the distribution of vertebrate 

 animals. A number of birds and mammals have been cited to 

 show how we may use our more or less detailed knowledge of their 

 ranges so as to demonstrate the operation of one or several out 

 of the many possible factors as limiters to distribution. The 

 method employed is one of examination, comparison and elim- 

 ination, applied to all parts of the margin of animals' ranges. 

 The range of any one animal must be examined at all points of 

 its periphery in order that all of the factors concerned may be 

 detected. One factor may constitute the barrier in one section 

 of the periphery of the range of a species, a totally different fac- 

 tor in another section. 



The results of the geometric ratio of reproduction would 

 bring about areas of occupancy in the shape of perfect circles. 

 But we never find such symmetrical ranges. The very fact that 

 the outlines of the ranges of animals are extremely irregular is 

 significant of the critical nature or inexorableness of the factors 

 which delimit them. These factors have to do with the evolu- 

 tion, persistence and extermination of species. 



Note that we always have to take into account, in attempting 

 to discern factors of limitation, the animal's own inherent struc- 

 tural equipment. This prescribes restriction at once in certain 

 regards. Keferring again to our list of suggested factors, we find 

 the long-emphasized ones of land to aquatic species and bodies 

 of water to terrestrial species really presenting an extreme mani- 

 festation of associational restriction. Food source, methods of 

 food-getting and safety refuges are involved. 



It is to be noted further that the factors are various and that 

 the most important factor for one species may prove of little 

 effect with another species. Species do not react uniformly to 

 the same environment. It is undoubtedly always a combination 

 of factors which accounts for an animal's geographic range in 

 all parts of the periphery of that range. It is most certainly 

 never one factor alone. No one will claim that temperature is 

 the only delimiting agent in controlling vertebrate distribution ; 

 nor could this claim be made for humidity alone, or for food 

 supply alone, or for safety of breeding-places alone. 



Given a large continuous area, however, as upon the North 



