On Some Interactions of Ore/an isms. 17 



a modifiability of their habits, numbers, and distribution, 

 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. 

 Startinji' with the general laws of multiplication and nat- 

 ural selection, it is first observed that every species of 

 plant or animal dependent upon living organic food is in- 

 terested to establish such a rate of reproduction as will, 

 first, meet all the drains to which it is itself subjected, and 

 still leave a sufficient i^rogeny to maintain its own num- 

 bers, 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 destructive plant or 

 animal are identical with the interests of its food supply. 



This common interest of the organism and 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-adjust- 

 ed rate of increase ; but, because of the immense number, 

 variability, and complexity of the forces involved, a com- 

 plete 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 depredating species and its 

 food. These oscillations of a species are both directly and 

 indirectly injurious to it, and tend, in various ways, to 

 diminish the average of its numbers, especially by lessen- 

 ing 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 order more or less completely. They tend 

 to become so adjusted to their food supplies as to appro- 

 priate only their surplus and excess. 



Hence, as a general thing, the real limits of a species are 

 not set by its organic environment, but by the inorganic ; 

 and the removal of the organic checks upon a species 

 would not finally diminish its average numbers. 



Among the external checks upon the oscillations of spe- 

 cies of insects, the most important are those predaceous 



