On Some Interactions of Organisms. 
17 
Summary. 
The argument and conclusions of this paper may be 
thus briefly recapitulated: — 
We find a mutual interdependence of organic groups 
and a modifiability of their habits, numbers, and distribu- 
tion which brings them under the control of man. We 
also see that, after the most violent disturbances of their 
internal relations, a favorable readjustment eventually 
occurs. Starting with the general laws of multiplication 
and natural selection, it is first observed that every spe- 
cies of plant or animal dependent upon living organic 
food is interested to establish such a rate of reproduction 
as will, first, meet all the drains to which it is itself sub- 
jected, and still leave a sufficient progeny to maintain its 
own numbers, and, second, leave a sufficient supply of its 
own food-species to keep them undiminished, year after 
year. That is, we find that the interests of any destruc- 
tive plant or animal are identical with the interests of its 
food supply. 
This common interest of the organisipand its organic 
food is continually promoted by natural selection, by 
which those that unduly weaken the sources of their own 
support are eventually crowded out by others with a 
better-adjusted rate of increase; but, because of the im- 
mense number, variability, and complexity of the forces 
involved, a complete adjustment is never reached. 
Whether the rate of multiplication of the food-producing 
species be relatively too great or relatively too small, the 
result is to cause an oscillation of numbers of both depre- 
dating species and its food. These oscillations of a spe- 
cies are both directly and indirectly injurious to it, and 
tend, in various ways, to diminish the average of its num- 
bers, especially by lessening the general average amount 
of the food available for it. By the operations of natural 
selection, therefore, widely oscillating species, thus 
placed at a marked disadvantage as compared with more 
. stable ones, are either eliminated, or else reduced to or- 
der more or less completely. They tend to become so 
adjusted to their food supplies as to appropriate only 
their surplus and excess. 
