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: 
Maximum Limit of 
Toleration 
Minimum Limit of 
Toleration Range of Optimum 
<_< | Habitat or center of distribution »—> 
Absent | Decreasing Greatest abundance Decreasing 
Absent 
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. 
