Man has risen from the ape chiefly through the action of natural selection. Any scheme of conscious race betterment, then, should carefully examine nature's method, to learn to what extent it is still acting, and to what extent it may better be supplanted or assisted by methods of man's own invention.
Natural selection operates in two ways: (1) through a selective death-rate and (2) through a selective birth-rate. The first of these forms has often been considered the whole of natural selection, but wrongly. The second steadily gains in importance as an organism rises in the scale of evolution; until in man it is likely soon to dwarf the lethal factor into insignificance. For it is evident that the appalling slaughter of all but a few of the individuals born, which one usually associates with the idea of natural selection, will take place only when the number of individuals born is very large. As the reproductive rate decreases, so does the death-rate, for a larger proportion of those born are able to find food and to escape enemies.
When considering man, one realizes at once that relatively few babies or adults starve to death. The selective death-rate therefore must include only those who are unable to escape their enemies; and while these enemies of the species, particularly certain microörganisms, still take a heavy toll from the race, the progress of science is likely to make it much smaller in the future.
The different aspects of natural selection may be classified as follows:
| { Lethal | { Sustentative | |
| { | { Non-sustentative | |
| Natural selection | { | |
| { Reproductive | { Sexual | |
| { | { Fecundal |
The lethal factor is the one which Darwin himself most emphasized. Obviously a race will be steadily improved, if the worst stock in it is cut off before it has a chance to reproduce, and if the best stock survives to perpetuate its kind. "This preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called natural selection, or the survival of the fittest," Darwin wrote; and he went on to show that the principal checks on increase were overcrowding, the difficulty of obtaining food, destruction by enemies, and the lethal effects of climate. These causes may be conveniently divided as in the above diagram, into sustentative and non-sustentative. The sustentative factor has acquired particular prominence in the human species, since Malthus wrote his essay on population—that essay which both Darwin and Wallace confess was the starting point of their discovery of natural selection.
There is a "constant tendency in all animated life to increase beyond the nourishment prepared for it," Malthus declared. "It is incontrovertibly true that there is no bound to the prolific plants and animals, but what is made by their crowding and interfering with each others' means of subsistence." His deduction is well known: that as man tends to increase in geometrical ratio, and can not hope to increase his food-supply more rapidly than in arithmetical ratio, the human race must eventually face starvation, unless the birth-rate be reduced.
Darwin was much impressed by this argument and ever since his time it has usually been the foundation for any discussion of natural selection. Nevertheless it is partly false for all animals, as one of the authors showed[52] some years ago, since a species which regularly eats up all the food in sight is rare indeed; and it is of very little racial importance in the present-day evolution of man. Scarcity of food may put sufficient pressure on him to cause emigration, but rarely death. The importance of Malthus' argument to eugenics is too slight to warrant further discussion.
When the non-sustentative forms of lethal selection are considered, it is seen very clearly that man is not exempt from the workings of this law. A non-sustentative form of natural selection takes place through the destruction of the individual by some adverse feature of the environment, such as excessive cold, or bacteria; or by bodily deficiency; and it is independent of mere food-supply. W. F. R. Weldon showed by a long series of measurements, for example, that as the harbor of Plymouth, England, kept getting muddier, the crabs which lived in it kept getting narrower; those with the greatest frontal breadth filtered the water entering their gills least effectively, and died.