PHYSIOLOGICAL ANIMAL GEOGRAPHY 599 
local condition. The evidence for the law of toleration as apply- 
ing to distribution is good so far as the local distribution is con- 
cerned and, since the same factors are involved in the geographic, 
there is no difficulty in the application of the law to geographic 
distribution also. The fact that in so far as our observation can 
go at present, the tiger beetles are found in similar conditions 
throughout their ranges, is also good evidence for the application 
of both the laws of minimum and toleration to geographic dis- 
tribution. In fact the law of minimum 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 makes the law of toler- 
ation 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. 
d. Application of the law of toleration to geographic distribu- 
tion. ‘The so-called centers of distribution are often only areas 
in which conditions are optimum for a considerable number of 
species (Transeau, 705; Adams, ’02 and ’05). The relation of the 
law to centers of distribution is shown in the diagram below; 
above the line is the scale of stimulation with the limits of toler- 
ation shown and below the parallel relation of the distribution and 
relative abundance. 
Minimum limit Range of Maximum limit 
of toleration optimum of toleration 
iL | » | | 
| | | 
—— Center of distribution —— 
Absent Decreasing Greatest abundance Decreasing Absent 
On account of the nature and distribution of climatic and 
vegetation conditions, it follows that as we pass in one direction 
from a center, one factor may fluctuate beyond the range of toler- 
ation of a species under consideration; but as we pass in another 
direction it is very likely to be a different factor. The divisions 
of Merriam’s zones into arid and humid portions is an illustration 
of this, and seems to constitute a begging of the temperature 
question. 
