ravel these mysteries, to the end that game management can 
use them for conservation purposes. 
Doves, grouse, and small birds need dust baths, pre- 
sumably for parasite control. ±-vossibly dustless rainy 
seasons have something to do with their diseases. If so, 
artificial covered dusting places might be a potent means 
of disease and parasite control. 
10. Subnormal and Abnormal Productivity I This review 
of the separate factors indicates that one of them often inter- 
acts with the others in such a manner as to control the result- 
ant population curve. In cases where we know or can guess at 
the controlling factor we ascribe subnormal or abnormal pro- 
ductivity to its influence. 
There Are, however, other cases where we do not even 
guess at the cause of subnormal or abnormal productivity% 
Antelope, for instance, have usually failed to "come back" 
in spite of totally closed seasons, and, in some cases, in 
spite of good range and good control of predators. To account 
for this failure writers on game conservation have hypothe- 
cated a " point of resistance" or minimum population, below 
which the species, for reasons unknown, fails to respond to 
protecti on. 
This conception of a "point of resistance" is valid 
and useful provided we keep in mind that it is probably not 
an exception to the general law governing productivity, but 
rather merely a convenient name for the instances where the 
controlling factors are unknown. To put the same thought in 
another way, a species which has fallen below its "point of 
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