86 The Lake as a Microcosm. 
were minute insect larve. When we estimate the myriads of small insects 
and Crustacea which these plants must appropriate during a year, to their 
own support, and consider the fact that these are of the kinds most useful as 
food for young fishes of nearly all descriptions, we must conclude that these 
plants compete with fishes for food and tend to keep down their number by 
diminishing the food resources of the young. The plants even have a certain 
advantage in this competition, since they are not strictly dependent on Ento- 
mostraca, as the fishes are, but sometimes take root, developing then but very 
few leaves and bladders. This probably happens under conditions unfavora- 
ble to their support by the other 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 remarkable than 
the steady balance of organic nature, which holds each species within the 
limits of a uniform average number, year after year, although every one is 
always doing its best to break across its boundaries, on every side. The repro- 
ductive rate is usually enormous and the struggle for existence is corresponding- 
ly 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 destruction 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 unending 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 exter- 
minated, 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 cannot maintain itself contin- 
uously, 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 certainly dwindle and disappear. 
On the other hand, the dependent species evidently 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 regularly 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 mul- 
tiplication, 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 ap- 
propriations 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 cer- 
tainly as the thrifty business man who lives within his income will finally 
dispossess his shiftless competitor who can never pay his debts, the well- 
