294 ENTOMOLOGY 



sitic plants. The ideal adjustment is one in which the repro- 

 ductive rate of each species should be so exactly adapted to its 

 food supply and to the various drains upon it that the species 

 preyed upon should normally produce an excess sufficient for 

 the species it supports. And this statement evidently applies 

 throughout the entire scale of being. Among all orders of 

 plants and animals, the ideal balance of Nature is one promo- 

 tive of the highest good of all the species. In this ideal state, 

 towards which Nature seems continually 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 fur- 

 nished. . . . 



" Exact adjustment is doubtless never reached anywhere, 

 even for a single year. It is usually closely approached in 

 primitive nature, but the chances are practically infinite against 

 its becoming really complete, and rnal-adjustment in some de- 

 gree 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 sup- 

 ply, the species will eventually yield to its more prolific compet- 

 itors 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 evidence of a fairly com- 

 plete 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. 



