INSECTS IN RELATION TO OTHER ANIMALS 22Q 



striving, every food-producing species of plant or animal would grow and 

 multiply at a rate sufficient to furnish the required amount of food, and 

 every depredating species would reproduce at a rate no higher than just 

 sufficient to appropriate the food thus furnished. . . . 



"Exact adjustment is doubtless never reached anywhere, ^ven for a 

 single year. It is usually closely approached in primitive nature, but 

 the chances are practically infinite against its becoming really complete, 

 and mal-adjustment in some degree is therefore the general rule. All 

 species must oscillate more or less." 



Professor Forbes then shows that oscillations are injurious to a species 

 and that the tendency of things is toward a healthy equilibrium. If the 

 rate of reproduction, as in a parasite for instance, is too small in relation 

 to the food supply, the species will eventually yield to its more prolific 

 competitors in the general struggle for existence. If, on the other hand, 

 its rate of multiplication is too high, the species will be at a disadvantage 

 in the search for food, as compared with better adjusted species, and must 

 again suffer. "The fact of survival is therefore usually sufficient evi- 

 dence of a fairly complete adjustment of the rate of reproduction to the 

 drains upon the species." . . . "We may be sure, therefore, that, as a 

 general rule, in the course of evolution, only those species have been able 

 to survive whose parasites, if any, were not prolific enough sensibly to 

 limit the numbers of their hosts for any length of time. 



"We notice incidentally that it is thus made unlikely that an injuri- 

 ous species can be exterminated, can even be permanently lessened in 

 numbers, by a parasite strictly dependent upon it, a conclusion which 

 remarkably diminishes the economic role of parasitism. The same line 

 of argument will, of course, apply, with slight modifications, to any 

 animal, or even to any plant dependent upon any other animal or any 

 other plant for existence. 



"It is a general truth, that those animals and plants are least likely 

 to oscillate widely which are preyed upon by the greatest number of 

 species, of the most varied habit. Then the occasional diminution of a 

 single enemy will not greatly affect them, as any consequent excess of 

 their own numbers will be largely cut down by their other enemies, and 

 especially as, in most cases, the backward oscillations of one set of ene- 

 mies will be neutralized by the forward oscillations of another set. But 

 by the operations of natural selection, most animals are compelled to 

 maintain a varied food habit, so that if one element fails, others may be 

 available. Thus each species preyed upon is likely to have a number of 

 enemies, which will assist each other in keeping it properly in check. 



