Part 7

While it is true that the number of individuals of a given species which live in a particular area is not exactly the same year in year out, being subject to small, and sometimes, as in the case of the aphides and cockchafers, to very great fluctuations, nevertheless we may assume that the _average number_ remains the same, that in the course of a century, or, let us say, of a thousand years, the number of mature individuals inhabiting the particular area remains the same. This, of course, only holds true on the supposition that there has been no great change in the external conditions of life during this period. But before Man began to interfere with nature, these external conditions would remain uniform for much longer periods than we have assumed. Let us call the average number of individuals occurring on such a uniform area, _the normal number_ of the species; this number will be determined in the first instance by the number of offspring that are annually brought forth, and secondly by the number that annually perish before reaching maturity. As the fertility of a species is a definite quantity, so also will its elimination be definite, or, as we may say, when the normal number under uniform conditions of life remains constant, the ratio of elimination will also remain constant. Each species is therefore subject to a perfectly definite ratio of elimination which remains on the average constant, and this is the reason why a species does not multiply beyond its normal number notwithstanding the great excess of the food-supply, and notwithstanding the fertility which, in all species, is sufficient to lead to boundless multiplication.

It is not difficult to calculate the ratio of elimination for a particular species, if one knows its rate of multiplication; for if the normal number remains constant, it follows that only two of all the offspring which a pair brings forth in the course of its life can attain to reproductive maturity, and that all the rest must perish.

Suppose, for instance, a pair of storks produced four young ones annually for twenty years, of these eighty young ones which are born within this period, on an average seventy-eight must perish, and only two can become mature animals. If more than two attained maturity the total number of storks would increase, and this is against the presupposition of constancy in the normal number. It is important, in reference to the fact on which we are now focusing our attention, that we should consider some other illustrations from the same point of view. The female trout yearly produces about 600 eggs; let us assume that it remains capable of reproduction for only ten years, then the elimination-number of the species will be 6,000 less two, that is, 5,998, for of the 6,000 eggs only two can become mature animals. But in the majority of fishes the ratio of extermination is enormously greater than this. Thus a female herring brings forth 40,000 eggs annually, the duration of life is estimated at ten years, and this means an elimination number of 400,000 less two, that is, 399,998. The carp produces 200,000 eggs a year, and the sturgeon two millions, and both species live long, and remain capable of reproduction for at least fifty years. But of all the 100 million eggs which are produced by the sturgeon, only two reach their full development and reproduce; all others perish prematurely.

But even with these examples we have not reached the highest elimination number, for many of the lower animals--not to speak of many plants--produce an even greater number of offspring. Leuwenhoek calculated the fertility of a thread-worm at sixty million eggs, and a tape-worm produces hardly less than 100 millions.

There exists, therefore, a constant relation between fertility and the ratio of elimination; the higher the latter is, the greater must the former be, if the species is to survive at all. The example of the tape-worm makes this very obvious, for here we can readily understand why the fertility must be so enormous, as we are aware of the long chain of chances on which the successful development of this animal depends. The common tape-worm of Man, _Tænia solium_, does not lay its eggs, they remain enclosed within one of the liberated joints or 'proglottides.' Only if this liberated joint or one of the embryos within it happens to be fortuitously eaten by a pig or other mammal can there be successful development, and even then under difficulties and possible failures, and not right away into adult animals, but first into microscopically minute larvæ which may bore their way into the walls of the intestine, or, if they are fortunate enough, may get into the blood-stream and be carried by it to a remote part of the body. There they develop into 'measles,' the so-called bladder-worms, within which the head of the tape-worm arises. But in order that this may become a complete and reproductive adult worm the pig must die, and the next step necessary is that a piece of the flesh of the infected first host must happen to be swallowed raw by a man or other mammal! Only then does the fortunate bladder-worm--swallowed with the flesh--attain the goal of its life, that is, a suitable place to mature in, the food-canal of a human being. It is obvious that countless eggs must be lost for one that succeeds in getting through the whole course of a development depending so greatly on chance. Hence the necessity for such enormous productivity of eggs.

In many cases the causes of elimination, which keep a species within due bounds, are very difficult to determine. Enemies, that is to say, other species which use the species in question as food, play an important rôle; often, however, the cause lies in the unfavourableness of external conditions, in chance, which is favourable only to one of a thousand. The oak would only require to produce one seed in the 500 years of its life, if it were certain that that one would grow into an oak-tree; but most of the little acorns are eaten up by pigs, squirrels, insects, &c., before they have had time to sprout, thousands fall on ground already thickly covered with growth where they cannot take root, and even if they do succeed in finding an unoccupied space in which to germinate, the young plants are still surrounded by a thousand dangers--the possibility of being devoured by many animals large and small, of being suffocated by the surrounding vegetation, and so on. We can thus understand, to some extent, though only approximately, why it is that the oak must year by year produce thousands of seeds in order that the species may maintain its normal number, and not be exterminated; for it is obvious that a constant, even though slow diminution of the normal number, a regular deficit, so to speak, can end in nothing else than the gradual extinction of the species.

But even this prodigality of seeds is not the greatest reach of fertility that we meet with in nature; it is, perhaps, amongst the simpler flowerless plants that we find the climax. It has been calculated that a single frond of the beautiful fern so common in our woods, _Aspidium filix mas_, produces about fourteen million spores. They serve to distribute the species, and are carried as motes by the wind, but comparatively few of the millions ever get the length of germinating at all, much less of attaining to full development into adult plants. Thus we see that the apparent prodigality of nature is a real necessity, an indispensable condition of the maintenance of the species; the fertility of each species is related to the actualities of elimination to which it is exposed. This is clearly seen when a species is placed under new and more favourable conditions of life, in which it has an abundant food-supply and few enemies. This was the case, for instance, with the horses introduced from Europe into South America, where they reverted to a feral state, and are now represented by herds of many thousands roaming the great grassy plains. If the small singing-birds of a region diminish in number, there is a great increase of caterpillars and other injurious insects which form part of their food-supply. The colossal destruction which the much-dreaded nun-moth from time to time brings about in our woods probably depends in part on the diminution of one or another of the many animals inimical to insects; but the occurrence of several years of weather-conditions favourable to the larvæ must also be taken into account. How enormously, indeed almost inconceivably, the number of larvæ may increase under favourable conditions is shown by such devastations as that in Prussia in 1856, when many square miles of forest were absolutely eaten up. The caterpillars were so numerous that even from some distance the falling excrement could be heard rustling like rain, and ten hundredweights of the eggs were collected, with an average of 20,000 eggs to the half-ounce!

But it would be a great mistake to conclude, from this enormous and sudden increase in the number of individuals, that the normal number of individuals is determined by the number of enemies alone. The average number of individuals in a species depends on many other conditions, especially on the extent of the available area, and on the amount of the food-supply in relation to the size of body in the species. I cannot dwell on this now, but I wish to point out that, for the continuance of a species, it is indifferent whether it is 'frequent' or 'rare,' if we presuppose that its normal number remains on an average constant for centuries, that is, that its fertility suffices to make good the continual losses through enemies and other causes of elimination. One would be inclined to conclude from such cases of sudden and enormous increase in the number of individuals as these caterpillar-blights, that enemies and other causes of destruction played the major part in the regulation of the normal number of the species. But this is only apparently the case. Enemies necessitate a certain fertility in the species on which they prey, so that the elimination in each generation may be made good; but the number of pairs capable of reproduction is not thereby decisively determined. We must not forget that the number of enemies is also, on the other hand, dependent on the number of victims, and that the normal number of enemies must rise and fall with that of the species preyed upon.

For this reason, such an enormous increase as that of the caterpillars cannot last long; it carries its corrective in itself. The appearance of the caterpillars in such enormous numbers in itself increases the host of their enemies; singing-birds, ichneumon-flies, beetle-grubs, and predaceous beetles find abundant and available food, and therefore reproduce and multiply so rapidly, that, with the help of the caterpillar's plant-enemies, especially the insect-destroying fungi, they soon reduce the caterpillars to their normal number, or even below it. But then the reverse process begins; the enemies of the caterpillars diminish because their food has become scarce, and their normal number is lowered, while that of the caterpillars gradually rises again.

When the number of foxes in a hunting district increases, the number of the hares that they prey upon diminishes, and, on the other hand, the decimating of the foxes by Man brings about an increase in the number of hares in the district. Under natural conditions, that is, without the intervention of Man, there would be a constant balancing of the numbers of hares and foxes, for every noteworthy increase of the hares would be followed by a similar increase of foxes, and this, in its turn, would diminish the number of hares, so that they would no longer suffice for the support of so many foxes, and these would decrease in number again, until the number of hares had again increased because of the lessened persecution and elimination. In nature the case is not quite so simple, because the fox does not live on hares alone, and the hare is not preyed upon only by the fox; but the illustration may serve to elucidate the point that a moving equilibrium is maintained between the species of a district, between persecutors and persecuted, in such a way that the number of individuals in the two species is always varying a little up and down, and that each influences the other so that a regulative process results. Throughout periods of considerable length the average remains the same; that is to say, a _normal number_ is established. This normal strength of population is the mean above and below which the number of individuals is constantly varying. It is, of course, seldom that the mutual influences and regulations are so simple as in the example given; usually several or even many species interact upon each other, and not beasts of prey and their victims alone, but the most diverse species of animals and plants, which do not stand in any obvious relation to one another at all. Moreover, the physical, and especially the climatic conditions, also cause the normal number of the species to rise and fall.

The inter-relations between species living together on the same area are so intricate that I should like to give two other illustrations. Let us first take Darwin's famous instance of the fertility of clover, which depends on the number of cats. It is of course only an imaginary one, but the facts it is based upon are quite correct. The number of cats living in a village to a certain extent determines the number of field-mice in the neighbourhood. These again destroy the nests of the humble-bees, which live in holes in the ground, and thus the number of humble-bees depends on that of the field-mice and cats. But the clover must be pollinated by insects if it is to produce fertile seed, and only the humble-bee has a proboscis long enough to effect the pollination. Therefore the quantity of clover-seed annually produced depends on the number of humble-bees, and ultimately upon the number of cats. And, as a matter of fact, humble-bees were introduced into New Zealand from England, because without them the clover would produce no fertile seeds.

On the grassy plains of Paraguay there are no wild cattle and horses, because of the presence of a fly which has a predilection for laying its eggs in the navel of the newly-born calves and foals, with the result that the calves or foals are killed by the emerging maggots. We may reasonably assume that the numerical strength of this fly-species depends on the distribution of insect-eating birds, whose numbers in turn are determined by certain beasts of prey. These again vary in number in relation to the extent of the forest-land, and this is determined by the number of ruminants which browse on the young growth of the woods (Darwin).

That forests can actually be totally destroyed by ruminants is proved by the case of the island of St. Helena among others. On its discovery the island was covered with thick wood, but in the course of 200 years it was transformed into a bare rock by goats and pigs, which devoured the young growth so completely that trees which were felled or which died were not replaced.

This point is vividly illustrated by Darwin's observation of a wide heath on which stood only a few groups of old pine-trees. The mere fencing in of a portion of the heath sufficed to call forth a thick growth of young seedling pines within the enclosure, and an examination of the open part of the heath revealed that the grazing cattle had eaten up all the young pine-trees which sprang from seed, and that again and again. In one small space thirty-two little trees stood concealed in the grass, and several of these showed as many as twenty-six yearly rings.

How definitely the number of individuals in different species living on the same area mutually limit and thereby regulate each other, Darwin sought to illustrate also by the case of the primitive forest, where the numerous species of plants occur, not mixed together irregularly, but in a definite proportion. We can find examples of the same kind wherever the plant-growth of a district has been left to itself. If we walk along the banks of our little river, the Dreisam, we see a wild confusion of the most diverse trees, shrubs and herbaceous plants. But, even though it cannot be demonstrated, we may be certain that these are represented in definite numerical proportions, dependent on the natural qualities and requirements of each species, on the number of their seeds and the facilities for their distribution, on the favourable or unfavourable season at which they ripen, and on their varying capacity for taking root in the worst ground, and springing quickly up, &c. They limit each other mutually, so that the whole flora of the river-bank will be made up of one per cent. of this species, one per cent. of that, and, it may be, five per cent. of a third, and the same combination will repeat itself in the same proportions on the banks of other rivers of our country in as far as the external conditions are the same. The same must be true of the fauna of such a plant-thicket; the animal species also limit one another mutually, and thereby regulate the number of individuals, which becomes relatively stable over any area on which the conditions remain the same. That is to say, a 'normal number' is attained and persists.

Thus we see that the capacity for boundless multiplication inherent in every species is limited by the co-existence of other species; there is, metaphorically speaking, a continuous struggle going on between species, plant and animal alike; each seeks as far as possible to multiply, and each is hemmed in by the others and as far as possible prevented from doing so. The 'struggle' is by no means only the _direct_ limitation of the number of individuals, which consists in the use of one species by another as food, as in beasts of prey and their victims, or locusts and plants; it is much more the indirect limitation--figuratively speaking, the struggle for space, for light, for moisture among plants, for food among animals. But all this, important as it is, does not yet exhaust the content of that 'struggle for existence' to which Darwin and Wallace ascribe the rôle of the breeder in the process of natural selection. The struggle, that is, the mutual limiting of species, may indeed restrict a species in its distribution, and may reduce its normal number possibly to nil. In other words, it may bring about extinction, but it cannot make a species other than it is. This can only be done by a struggle within the limits of the species itself, and this struggle is due to the fact that of the numerous offspring, on an average those survive--that is, attain to reproduction--which are the most fit, whose constitution makes it most possible for them to overcome the difficulties and dangers of life, and so to reach maturity. We see, in fact, that a large percentage of each generation in all species always perishes before attaining maturity. If, then, the decision as to which is to perish and which is to reach maturity is _not a matter of chance alone_, but is in part due to the constitution of the growing individual; if the 'fittest' do _on the average_ survive, and the 'least fit' are on the average eliminated, we have here a process of selection entirely comparable to that of artificial selection, and one whose result must be the 'improvement' of the species, whether that depends on one set of characters or on another. The victorious qualities, which earlier were peculiar to certain individuals, must gradually become the common property of the species, if in each generation the individuals which attained to reproduction all possessed them, and thus could transmit them to their progeny. But those of the descendants which did not inherit them would again be at a disadvantage in the struggle for existence, or rather for reaching maturity, if in each generation a higher percentage of individuals which possess these characters reach maturity than of those which do not possess them. This percentage must increase in each generation, because, in each, natural selection again chooses out the fittest, and it must finally rise to 100 per cent., that is to say, none but individuals of this fittest type will be left surviving.

This does not yet exhaust the process, however, for we can infer from the results of artificial breed-forming that the selected characters may intensify from generation to generation, and that they will continue to do so as long as it gives them any advantage in the struggle for existence, for so long will it lead to the more frequent survival of its possessors. The increase will only stop when it has reached the highest degree of usefulness, and in this way new characters may be formed, just as, in artificial selection, the short upward-turning feathers of the Jacobin pigeon have been intensified into the peruke, a feather canopy covering the head.

A few examples of natural selection will make the process clearer. Our hare is well secured from discovery by his fur of mixed brown, yellow, white, and black, when he cowers in his form among the dry leaves of the underwood. It is easy to pass close to him without seeing him. But if the ground and the bushes are covered with snow, he contrasts conspicuously with them. Suppose, now, that our climate became colder, and that the winter brought lasting snow, the hares which had the largest mixture of white in their fur would have an advantage in their 'struggle for existence' over their darker fellows; they would be less easily discovered by their enemies--the fox, the badger, the horned owl, and the wild cat. Of the numerous hares which would annually become the prey of these enemies, there would be, on an average, more dark than light individuals. The percentage of light-coloured hares would, therefore, increase from generation to generation, and the longer the winter the keener would be the selection between dark and light hares, until finally none but light ones would remain. At the same time, the colour of the hares would become increasingly light, first, because it would happen more and more frequently that two light hares would pair, and secondly, because, after a time, the struggle for existence would no longer be between light and dark hares, but between light hares and still lighter ones. Thus ultimately a race of white hares would arise, as has actually happened in the Arctic regions and on the Alps.

Or let us think of a herbaceous plant, in appearance something like a belladonna, rich in leaves and very juicy, but not poisonous. It would doubtless be a favourite food with the animals of the forest, and it would not, therefore, attain to more than a sparse occurrence, since few of the individuals would be able to form seeds. But now let us assume that a stuff of very unpleasant taste develops in the stem and leaves of some of the individuals, as may easily happen through very slight changes in the chemical metabolism of the plant, what, then, could result but that such individuals would be less readily eaten than the others? A process of selection must, therefore, ensue, and the unpleasant-tasting specimens of the plant would be much more frequently spared, and consequently would bear seed much oftener than the palatable ones. Thus the number of unpalatable plants would increase from year to year. If the stuff in question were not only unpalatable but poisonous, or gradually became so, a plant would in time be evolved which would be absolutely safe from being devoured by animals, just as the deadly nightshade (_Atropa belladonna_) actually is.