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 1s 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 mal-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 1s 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. 3 ...4>"" Wewmay ibe sure, “thereiore, that, acac 
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. 
