many more individuals are born than can possibly survive. The result is that those born with certain weaknesses or under unfavorable conditions are the ones which are most likely to die, while those possessing greater strength or born under favorable conditions are the ones most likely to live. Hence it is that there tends to be a survival of the fit. to survive.

Nature, so to say, selects the best

It is a self-evident fact that the amount of space upon the earth is limited. At first thought it is not so evident that living things tend to multiply in geometrical progression. But the truth of this principle is easily demonstrated. Romanes tells us that if the progeny of a single pair of elephants, which are the slowest breeding of animals, were allowed to reach maturity and propagate, in 750 years there would be living 19,000,000 descendants.1 Professor Metcalf has computed the following table based upon the rate of increase of the common robin. Supposing that the yearly offspring of each pair of robins is four on the average, which is below the usual number, then a single pair of birds would have four young in the first generation. The second year they would have four more young, and their young of the first year, mating, would have eight young, four for each of the two pairs. In twenty years the descendants of the original pair would number over twenty billion! 2

This should make it clear that the earth could not support the progeny of even a single species if the natural increase were allowed to go unchecked.

But in the case of the robins, more birds die each year than live because we find that the number remains con

1 Romanes, G. J.-Darwin and After Darwin, I The Darwinian Theory, 1901, p. 261.

2 Metcalf, op. cit., p. 14.

stant from year to year. There seems to be no great fluctuation in the number of any species from year to year. Yet this apparently high death-rate of robins is surpassed by that of many other species. Among many fishes the yearly death-rate is two hundred and fifty thousand times as great as the permanent population, since on the average only one male and one female out of the half million of young survive to take the place of their parents and keep the number of individuals in the species up to the usual mark. For every starfish living nearly half a million die each year. Indeed, taking organic nature as a whole probably not one in a thousand young is allowed to survive to the age of reproduction.*

While this law applies to the lower forms of life, plants and animals, one might say that men are not subject to it. It is true that the rigors of the crude struggle have been somewhat modified by man's greater cunning and forethought, but the law holds for men just as it does 4 Romanes, op cit., p. 262.

3 Ibid.,
p. 15.

for snails and pansies, though in a slightly lessened degree. In the registration area of the United States in the year 1910, there were recorded 805,412 deaths from all causes. When we examine the number of deaths at different age periods we find that 26.98 per cent. of those who died were under 5 years of age. At no other five year period of life was the per cent. higher than 6.2, and this was at the five year age period 65-69 years. The following table shows precisely what the situation

5 See Statistical Abstract of the United States, 1911, p.

77.

This table, especially in the large infantile mortality, is sufficient to show that the struggle for life is not a phenomenon peculiar to lower animals. The high mortality in early years is evidence of the selective death-rate. The 26.98 per cent. of deaths under 5 years of age indicates the extinction of the less fit. The weaker children and those born under unfavorable circumstances are more likely to die before they are five years of age than are the stronger children or those born under more favorable circumstances. Thus it is that Nature selects the fittest

to survive.

Because of the limited amount of food and space upon the earth and because many more individuals are born than can survive, there is a perpetual battle for life going on among all the individuals of any generation. In this terrible struggle for existence what individuals will be victorious and live? Obviously those best fitted to live, in whatever respect or respects their superiority of fitness may consist. These favored individuals transmit to their progeny their advantageous qualities. According to the laws of heredity the characters of the surviving generation are inherited by their offspring. It therefore follows that the individuals composing each successive generation have a general tendency to be better suited to their surroundings than were their forefathers. And so it is that since most of the weaklings die in infancy, the perpetuation of the race is by the "flower of the flock" and the species tends to grow stronger. This is Darwin's great theory of Natural Selection, or selection by nature, for, out of the thousands who die, the thousandth individual who does survive in the battle for existence is on the whole the one best fitted to do so. If now, in any generation some new

and beneficial qualities happen to arise as slight variations from the ancestral type, they will (other things permitting), be seized upon by natural selection, and being transmitted by heredity to subsequent generations, will be added to the previously existing type. This then, is natural selection or the survival of the fittest, the one term referring mainly to the process, the other to the result.

The process is analogous to that by which the gardener and the cattle-breeder bring about their wonderful results. Just as these men, by always "selecting" their best individuals to breed from, slowly but continuously improve their stock, so Nature by a process of "selection," slowly but continuously makes the various species of plants and animals better suited to the conditions of their life. What the skill of Luther Burbank has accomplished in the course of a few generations, Nature takes years or even centuries of experimentation to produce. By artificial selection, man works on external characters irregularly and imperfectly for a short time. Nature works on the whole machinery of life by consistent accumulation during whole geological epochs. Silently and insensibly working, natural selection is daily and hourly scrutinizing the slightest variation, rejecting those that are bad, preserving and adding up all that are good."

Under natural conditions there is an endless range of variation. We have seen in chapter I how like tends to beget like, but that although the offspring is similar to the parent there is never precise reduplication. There is latitude allowed for individual variation. The individual differences are due to age, sex, modification, and

6 Thomson & Geddes, op. cit., p. 156.