Field Tests of Theories Concerning Distributional Control

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 elimination, 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 factor 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 inconvenience of the factors which delimit them. These factors have to do with the evolution, 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 structural equipment. This prescribes restriction at once in certain regards. Referring 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 manifestation 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