As usual, Darwin’s disciples have failed to improve upon the account he gave of the nature of the struggle for existence. This is set forth in Chapter III. of the Origin of Species.
“The causes,” writes Darwin (new edition, p. 83), “which check the natural tendency of each species to increase in number are most obscure. Look at the most vigorous species; by as much as it swarms in numbers, by so much will it tend to increase still further. We know not exactly what the checks are even in a single instance.” This is perfectly true. Nevertheless elaborate theories of protective and warning colouration and mimicry have been built up on the tacit assumption that the checks to the multiplication of all, or nearly all, species are the creatures which prey upon them. Possibly no Wallaceian asserts this in so many words, but it is a logical deduction from the excessive prominence each one gives to the various theories of animal colouration; for, if the chief foes of an organism are not the creatures which prey upon it, how can the particular shade and pattern of its coat be of such paramount importance to it?
Checks on Increase
We shall endeavour to show that there are checks on the increase of a species far more potent than the devastation caused by those creatures which feed upon it. Let us, however, first briefly set forth some of the checks on the multiplication of organisms which Darwin mentions in the Origin of Species.
“Eggs, or very young animals,” he says, “seem generally to suffer the most, but this is not invariably the case.” This is, as we have already insisted, a most important point to be borne in mind, especially when considering the various current theories of animal colouration. When once the average animal has become adult its chances of survival are enormously increased.
A second check mentioned by Darwin is the limitation of food supply. “The amount of food for each species,” he writes (p. 84), “of course gives the extreme limit to which each can increase; but very frequently it is not the obtaining food, but the serving as prey to other animals, which determines the average numbers of a species. Thus there seems to be little doubt that the stock of partridges, grouse, and hares on any large estate depends chiefly on the destruction of vermin. . . . On the other hand, in some cases, as with the elephant and rhinoceros, none are destroyed by beasts of prey.”
We are inclined to think that neither the food limit nor the beasts of prey are a very important check on the multiplication of organisms. The lion, for example, was never so numerous as to reach the limit of its food supply. Before the white man obtained a foothold in Africa vast herds of herbivores were to be seen in those districts where lions were most plentiful. This is a most important fact, for, if the numbers of a species are not determined by those of the animals that prey upon it, the particular colour of an organism is probably not of any direct importance to it. This cuts away the foundation of some of the generally accepted theories of animal colouration.
“Climate,” writes Darwin (p. 84), “plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or drought seem to be the most effective of all checks. I estimated (chiefly from the greatly reduced numbers of nests in the spring) that the winter of 1854-55 destroyed four-fifths of the birds in my own grounds, and this is a tremendous destruction when we remember that 10 per cent. is an extraordinarily severe mortality from epidemics with man.”
In our opinion, Darwin did not lay nearly enough stress upon the importance of climate as a check on the increase of species. We have seen that he stated his belief that it is the most effective of all checks. But even this is not a sufficiently strong statement of the case. It seems to us that before this check all other checks pale into insignificance.
Darwin failed to notice the potent effects of damp. Damp is more injurious to most species than even cold or drought, as every one who has tried to keep birds in England knows. All entomologists are aware how harmful damp is to insects. Caterpillars seem to take cover under leaves to avoid damp rather than to hide themselves from birds, since these make a point, when searching for insects, of invariably looking carefully under leaves.