LAW OF TOLERATION 303 



The success of a species, its numbers, sometimes its size, etc., are 

 determined largely by the degree of deviation of a single factor (or 

 factors) from the range of optimum of the species. It is obvious that 

 the cause of the fluctuation might be, for example, moisture due to 

 (climatic) deficiency in rainfall, or rapid run-off, due to steep slope. 

 The evidence for the application of the law of toleration to local distribu- 

 tion is good. Since the same factors are involved in the "geographic" 

 or more extensive distribution, there is no difficulty in the application 

 of the law to such distribution also, for, to assume that the law is not 

 applicable is to assume that animals distinguish between the causes 

 which lie back of the changes in physical factors by which they are affected. 

 The fact that, in so far as our observation can go at present, most animals 

 are found in similar conditions throughout their ranges is also good 

 evidence for the application of both the laws of minimum and toleration 

 to problems of geographic range. In fact, the law of minimum (see p. 68) 

 is but a special case of the law of toleration. Combinations of the factors 

 which fall under the law of minimum may be made, which make the law 

 of toleration apply quite generally. For example, food and excretory 

 products may be taken together as constituting a single factor. From 

 this point of view the law of toleration applies, the food acting on the 

 minimum side, excretory products on the maximum. 



4. APPLICATION OF THE LAW OF TOLERATION TO DISTRIBUTION (55) 



As has already been implied, the locality or region of optimum, or 

 the locality or region in which the animal is most nearly in physiological 

 equilibrium, is called the habitat (ecological optimum) when it refers 

 to ecological or local distribution, and the center of distribution when it 

 refers to extensive areas. The so-called centers of distribution are 

 often only areas in which conditions are optimum for a considerable 

 number of species. The distribution and number of individuals of any 

 species may be graphically represented as below: 



On account of the nature and distribution of climatic and vegetational 

 conditions, it follows that as we pass in one direction from a center, one 

 factor may fluctuate beyond the range of toleration of a species under 

 consideration; but as we pass in another direction the fluctuating 

 factor is very likely to be different. 



