after twenty-four hours of development under ordinarily favorable conditions.  Though a single rodlike individual taken as a starting-point may be less than one five-thousandth of an inch in length, under natural circumstances it multiplies at a rate which within five days would cause its descendants to fill all the oceans to the depth of one mile.  This is a fact, not a conjecture; the size of one organism is known, and the rate of its natural increase is known, so that it is merely a matter of simple arithmetic to find out what the result would be in a given time.

Even in the case of those animals that reproduce more slowly, an overcrowding of the earth would follow in a very short time.  Darwin wrote that even the slow-breeding human species had doubled in the preceding quarter century.  An elephant normally lives to the age of one hundred years; it begins to breed at the age of thirty, and usually produces six young by the time it is ninety.  Beginning with a single pair of elephants and assuming that each individual born should live a complete life, only eight hundred years would be requisite to produce nineteen million elephants; a century or two more and there would be no standing room for the latest generation of elephants.  It is only too obvious that such a result is not realized in nature, but it is on account of other natural checks, and not because the natural rate of reproductive increase is anything but excessive.

The third element of the process of natural selection is the struggle for existence which is to a large extent the direct consequence of over-multiplication.  Because nature brings more individuals into existence than it can support, every animal is involved in many-sided battles with countless foes, and the victory is sometimes with one and sometimes with another participant in the conflict.  A survivor turns from one vanquished enemy only to find itself engaged in mortal combat with other attacking forces.  Wherever we look, we find evidence of an unceasing struggle for life, and an apparently peaceful meadow or pond is often the scene of fierce battles and tragic death that escape our notice only because the contending armies are dumb.

A community of ants, often comprising more individuals than an entire European state, depends for its national existence upon its ability to prevail over other communities with which it may engage in sanguinary wars where the losses of a single battle may exceed those of Gettysburg.  The developing conger-eels find a host of enemies which greatly deplete their numbers before they can grow even into infancy.  An annual plant does not produce a million living offspring in twenty years because seeds do not always fall upon favorable soil, nor do they always receive the proper amount of sunlight and moisture, or escape the eye of birds and other seed-eating animals.  These three illustrations bring out the fact that there are three classes of