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method. These simple instances will suffice to illustrate the intimate 

 way in which the living forms of a lake are united. 



Perhaps no phenomenon of life in such a situation is more remark- 

 able than the steady balance of organic nature, which holds each species 

 within the limits of a uniform average number, year after year, although 

 each one is always doing its best to break across boundaries on every 

 side. The reproductive rate is usually enormous and the struggle for 

 existence is correspondingly severe. Every animal within these bounds 

 has its enemies, and Nature seems to have taxed her skill and ingenuity 

 to the utmost to furnish these enemies with contrivances for the destruc- 

 tion of their prey in myriads. For every defensive device with which 

 she has armed an animal, she has invented a still more effective apparatus 

 of destruction and bestowed it upon some foe, thus striving with un- 

 ending pertinacity to outwit herself ; and yet life does not perish in the 

 lake, nor even oscillate to any considerable degree, but on the contrary 

 the little community secluded here is as prosperous as if its state were 

 one of profound and perpetual peace. Although every species has to 

 fight its way inch by inch from the egg to maturity, yet no species is 

 exterminated, but each is maintained at a regular average number which 

 we shall find good reason to believe is the greatest for which there is, 

 year after year, a sufficient supply of food. 



I will bring this paper to a close, already too long postponed, by 

 endeavoring to show how this beneficent order is maintained in the 

 midst of a conflict seemingly so lawless. 



It is a self-evident proposition that a species can not maintain itself 

 continuously, year after year, unless its birth-rate at least equals its death- 

 rate. If it is preyed upon by another species, it must produce regularly 

 an excess of individuals for destruction, or else it must certainlv 

 dwindle and disappear. On the other hand, the dependent species evi- 

 dently must not appropriate, on an average, any more than the surplus 

 and excess of individuals upon which it preys, for if it does so it 

 will continuously diminish its own food supply, and thus indirectly but 

 surely exterminate itself. The interests of both parties will therefore be 

 best served by an adjustment of their respective rates of multiplication 

 such that the species devoured shall furnish an excess of numbers to 

 supply the wants of the devourer, and that the latter shall confine its 

 appropriations to the excess thus furnished. We thus see that there 

 is really a close community of interest between these two seemingly 

 deadly foes. 



And next we note that this common interest is promoted by the 

 process of natural selection ; for it is the great office of this process to 

 eliminate the unfit. If two species standing to each other in the relation 

 of hunter and prey are or become badly adjusted in respect to their 

 rates of increase, so that the one preyed upon is kept very far below 

 the normal number which might find food, even if they do not presently 

 obliterate each other the pair are placed at a disadvantage in the battle 

 for life, and must suffer accordingly. Just as certainly as the thrifty 



