Part 33

All organic beings are exposed to severe competition. Nothing is easier than to admit in words the truth of the universal struggle for life, or more difficult than constantly to bear this conclusion, which has been reached through the investigations and researches of De Candolle, Lyell, Herbert, Darwin and others, in mind. Unless, however, it be thoroughly ingrained in the mind, the whole economy of nature, with every fact on distribution, rarity, abundance, extinction and variation, will be but dimly perceived or quite misunderstood. We behold the face of nature radiant with gladness, and food everywhere in excessive abundance, but we do not see that the birds which are happily singing round us mostly live on insects or seeds, and are thus constantly destroying life, or we fail to remember how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey. Yes, we do not always bear in mind that, though food may now be superabundant, it is not so at all seasons of each recurring year. The term, Struggle for Existence, must be used in a large and metaphorical sense. It must be construed to include the dependence of one being on another, and also not only the life of the individual but also its success in leaving offspring. Two carnivores, in a time of scarcity of food, may be truly said to struggle with each other for maintenance of life. But a plant on the edge of a desert is said to struggle for life against the drought, though, properly speaking, it is dependent for its existence upon the moisture. A plant, however, that annually produces many thousand seeds of which on an average only one comes to maturity, may in a much truer sense be said to struggle with the plants of the same and other kinds which already invest the ground. While the mistletoe is dependent on the apple and some other trees, yet it cannot be said, unless in a far-fetched sense, to struggle with these trees, for, if too many of these parasites are found upon the same tree, it will certainly languish and die. Several seedling mistletoes, however, growing close together upon the same branch, may more truly be said to struggle with each other.

From the high rate at which all organic beings tend to increase, there must inevitably follow a Struggle for Existence. Every being which, during its natural lifetime, produces several eggs or seeds, must necessarily suffer destruction during some part of that period, and during some season or occasional year, otherwise, on the principle of Geometrical Increase, its numbers would become so inordinately excessive that no country would be able to support its product. Therefore, as more individuals are produced than can possibly survive, there must be in every case a Struggle for Existence, either one individual struggling with another of the same kind, or with individuals of distinct kinds or species, or with the conditions of the environment. This is the doctrine of Malthus applied with manifold force to the entire vegetable and animal kingdoms. Although some species may be now increasing at a very high rate in numbers, yet all cannot do so, for the earth would not be able to contain them. Slow-breeding man has doubled in twenty-five years, and should he go on at this rate for a few thousand years, there would literally not be standing room for his progeny. It has been calculated that, if an annual plant produced only two seeds, and their seedlings next year produced two, and the same rate of increase was kept up for twenty years, there would be a million of plants as the result. Even the elephant, which is reckoned the slowest breeder of all known animals, would after a period of from seven hundred and forty to seven hundred and fifty years leave nearly nineteen million elephants as descendants from the first pair.

Much better evidence than mere theoretical calculations are not wanting on this subject. Instances are recorded of the astonishingly rapid increase of various animals in a state of nature, when conditions have been favorable to them, during two or three succeeding seasons. More striking, however, is the evidence from domestic animals that have run wild in several parts of the world. Were not the statements of the rate of increase of cattle and horses in South America, and latterly in Australia, where millions now abound, well authenticated, they would have been incredible. Cases could be mentioned of introduced plants that have become quite common throughout entire islands in a period of less than twelve years. Several of these plants, the cardoon and a rare thistle, which were introduced from Europe, clothe square leagues of the surface of the wide plains of the La Plata almost to the exclusion of all other plants; and there are plants which now range in India, from Cape Comorin to the Himalaya, which have been imported from America since its discovery. In all such cases, and endless instances could be adduced, no intelligent person supposes that their fertility has been increased in any sensible degree by change of habitat, the obvious explanation being that the conditions of environment have been very favorable, and that there has consequently been less destruction of old and young, and that nearly all the latter have been enabled to breed. The extraordinarily rapid increase and wide diffusion of naturalized productions in new homes, a result which never fails to evoke surprise, is only to be explained on the principle of the Geometrical Ratio of Increase. As in nature almost every plant produces seed, and there are very few animals that do not annually pair, therefore we can confidently assert that all plants and animals are tending to increase in a geometrical ratio; that all would most rapidly stock every station in which they could in any way exist, and that the tendency to increase must be checked by destruction at some period of life. Among our larger domestic animals we see no great destruction falling on them. We forget that thousands are annually slaughtered for food, and that in a natural state an equal number would have to be disposed of in some way or other. Between organisms which annually produce seeds or eggs by the thousands, and those which produce extremely few, the only difference is that the slow breeders would require a few more years to people, under favorable conditions, a whole district, let it be ever so large. But a couple of eggs are laid by the condor, while the ostrich lays a score. Yet in the same country the condor may be the more abundant of the two. The Fulmer petrel lays but a single egg, yet it is believed to be the most numerous bird in the world. A large number of eggs is of some importance to those species which depend upon a rapidly-fluctuating quantity of food, for it permits them to increase rapidly in number; but the real importance of a large number of eggs or seeds is to make up for the great destruction that goes on at some period of life, and this period in the vast majority of cases is an early one. If an animal can in any way protect its own eggs or young, a small number may be produced, and the average stock be kept up; but if many eggs or young are destroyed, then many must be produced or the species will become extinct. Therefore, the average number of any animal or plant depends, though only indirectly, upon the number of its eggs or seeds. We should never forget, in taking a survey of nature, that every single organic being around us may be said to be striving to the utmost to augment its members; that each lives by a struggle at some period of its existence, and that heavy destruction falls either on the young or old during each generation or at recurrent intervals. Let any check be lightened, or the destruction be mitigated ever so little, and the number of the species will almost instantaneously increase to any extent.

But of the nature of the checks to increase we know little, although this subject has been very ably treated by writers of eminence. Eggs or very young animals seem generally to suffer the most, but this is not invariably the case. While there is a vast destruction of the seeds of plants, but it is the seedlings which are believed to suffer the greatest, from germinating in ground already thickly stocked with other plants, and from being destroyed in large numbers by various enemies. The amount of food for each species of course determines the extreme limit to which each can increase, but very often it is not the obtaining of food, but the serving as prey to other animals which fixes the average number of a species. Thus there seems to be little doubt that the stock of partridges, grouse and hares on any large estate depends mainly on the destruction of vermin. Were not a single head of game shot during the next twenty years in England, says Darwin in substance, and no vermin were at the same time destroyed, there would in all probability be less game than at present exists, although hundreds of thousands of game animals are now annually killed for the market. In some cases, on the other hand, as in the case of the elephant, none are destroyed by beasts of prey, for even the tiger in India, bold and venturesome as he is known to be, rarely dares to attack a young elephant protected by its mother. Climate, also, plays an important part in determining the average number of a species, and periodical seasons of extreme cold or drought are seemingly the most effective checks of all. The action of climate appears at first sight to be altogether independent of the Struggle for Existence; but in so far as it chiefly acts in the reduction of food, it brings on the most severe struggle between the individuals, whether of the same or different species, which subsist on the same kind of fare. Even when climate, extreme cold for example, acts directly, it will be the least vigorous animals, or those which have been the poorest fed through the advancing winter, that will suffer the greatest. This will be most readily seen from what we shall now relate. When we travel from south to north, or from a damp region to a dry, we invariably see some species getting rarer and rarer by degrees, and finally disappearing. Change of climate being conspicuous, we are inclined to ascribe the entire effect to its direct action, but this is a false interpretation of the phenomenon, for we fail to remember that each species, even where it most prevails, is constantly suffering enormous destruction at some period of its existence, from enemies or competitors for the same station and food; and if these enemies or competitors be the least favored by any slight change of climate, they will necessarily increase in numbers, while the other species, each area being already stocked with inhabitants, will correspondingly decrease. And when we travel southward and see a species decreasing in numbers, we may feel reasonably sure that the cause lies quite as much in other species being favored as in this being hurt. So it is when we travel northward, though in a less degree. When we go northward, or when we ascend a mountain, we far oftener meet with stunted forms, due to the directly injurious action of climate, than we do when we go southward or descend a mountain. When, however, we reach the Arctic regions, or explore snow-capped summits, or absolute deserts, we perceive the struggle for life to be almost exclusively with the elements.

That climate operates mainly, but indirectly, in favoring other species, may be clearly seen in the prodigious numbers of garden plants that can thoroughly well endure our climate, but which can never become naturalized, inasmuch as they cannot compete with native vegetation nor resist destruction by native animals.

When a species, owing to highly favorable conditions, increases inordinately in numbers in a small tract of country, epidemics, especially in game animals, often occur, and here we have a limiting check independent of the Struggle for Existence. But some of these so-called epidemics appear to be due to parasitic worms, which have from some cause, possibly in part through ease of diffusion among the crowded animals, been disproportionately favored, and here comes in a sort of struggle between the parasite and its more illustrious prey.

But, on the other hand, as is frequently the case, a large stock of individuals of the same species, relatively to the number of its enemies, is absolutely essential to its preservation. We thus see how it is possible to raise with ease a plentiful supply of corn in our fields, because the seeds are greatly in excess of the number of birds which feed thereon. Nor can the birds, though blessed with a superabundance of food at this one season, increase in number in proportion to the supply of seed, as their numbers are checked during the winter. Any one, however, who has made the experiment, knows how troublesome it is to get seed from a few wheat or other such plants sown broad-cast in a garden. Some singular facts in nature, such as that of very rare plants being sometimes extremely abundant in the few spots where they do occur, and that of some social plants being social, or abounding in individuals, even on the extreme confines of their range, are readily explainable by this view of the necessity of a large stock of the same species for its preservation, for in such cases we may believe that a plant could only exist where the conditions of its life were so favorable that many could exist together and thus save the species from extinction.

Complex and varied are the checks and relations between organic beings which have to struggle together in the same country. In the case of every species, many different checks, some very complicated and unintelligible to man at present, acting at different periods of life, and during different seasons or years, come into play, some one check or some few being generally the most powerful, but all concurring in determining the average number or even the existence of the species. Widely-different checks sometimes act on the same species in different districts. Looking at the plants and bushes that clothe an entangled bank, we are tempted to ascribe their proportional numbers and kinds to what we call chance. But this is a very false view to take of the matter. Chance has no part in such things. They follow in obedience to laws of which we know comparatively little. When an American forest is cut down a very different vegetation springs up. Ancient Indian ruins have been observed in the southern parts of the United States, which must in former times have been cleared of trees, but which now display the same beautiful diversity and proportion of kinds as are now found in the surrounding virgin forest. What a struggle must have gone on during long centuries between the several kinds of trees, each annually scattering its seeds by the thousand, and what a war between insect and insect, and between insects, snails and other animals with birds and beasts of prey, all striving to increase, all feeding on each other, or on the trees, their seeds and their seedlings, or on the other plants which once clothed the soil, and thus checked the growth of the trees! It is easier to account for the fall of an apple from a tree, or the descent of a stone to the earth when hurled into the air, than to account for the action and reaction of the innumerable plants and animals that have determined in the course of untold centuries the proportional numbers and kinds of trees that are now found growing on these old Indian ruins. But the struggle will almost invariably be the severest between individuals of the same species, for they frequent the same districts, require the same food and are exposed to the same dangers. In the case of varieties of the same species, the struggle will generally be almost equally severe. If several varieties of wheat be sown together, and the mixed seed be re-sown, some of the varieties which best suit the soil or climate, or are naturally the most fertile, will beat the others and so yield more seed, and will consequently in a few years supplant the others. Such extremely-close varieties as the variously-colored sweet-peas must be separately harvested each year, and the seed mixed in due proportion, or the weaker kinds will steadily decrease in number and disappear. So, again, with the varieties of sheep. Certain mountain-varieties will starve out other mountain-varieties, so that they cannot be kept together. Similar results have followed from keeping together different varieties of the medicinal leech. In view of all that has been said, it is questionable whether the varieties of any of our domestic plants and animals have so exactly the same vigor, constitution and habits that the original proportions of a mixed stock could be kept up for a half-dozen generations if they were permitted to struggle together like beings in a state of nature, if the seed or young were not annually assorted.

Species of the same genus having usually, though not invariably, much similarity in habits and constitution, and always in structure, the struggle will be more severe between species of the same genus, where they come into competition with each other, than between species of distinct genera. One species of swallow has caused in certain parts of the United States the decrease of another species, just as the missel-thrush in parts of Scotland has caused the decrease of the song-thrush. The small Asiatic cockroach has everywhere in Russia driven before it its great congener, and the imported European hive-bee is rapidly exterminating in Australia the small, stingless bee, indigenous to the country. Hundreds of such cases might be cited, but we forbear. We can clearly see why the competition should be most severe between allied forms, which fill nearly the same place in the economy of nature; but it is perhaps not possible to individualize a case and say with preciseness why such species has been victorious over another in the battle of life. That the structure of every organic being is related, in the most essential yet often hidden manner to that of all the other organisms with which it comes into competition for food or residence, or from which it has to escape, or on which it preys, is a corollary of the highest importance deducible from the foregoing remarks. Very obvious is this in the structure of the teeth and talons of the tiger, and in that of the legs and claws of the parasite which clings to the hair on the tiger’s body. But in the beautifully-plumed seed of the dandelion and the flattened and fringed legs of the water-beetle the relation seems at first restricted to the elements of air and water, yet the advantage of plumed seeds undoubtedly stands in the most intimate relation to the land, being already densely clothed with other plants, so that the seeds may be widely diffused and fall on unoccupied ground, while in the water-beetle, the structure of its legs, so admirably adapted for diving, allows it to compete with other aquatic insects, to hunt for its own prey and to escape destruction by other predaceous animals. All organic beings, it will thus be seen, are not only striving to increase in numbers, but are called upon some time in their lives to struggle for existence or to suffer serious if not utter destruction. When we reflect on this struggle, we can console ourselves with the full belief that this war of nature is not incessant, that no fear is felt, that death is generally sudden, and that the vigorous, healthy and happy survive and multiply.

Seeing what a potent influence the principle of Selection has in the hands of man, in regard to variation, can it be applied in nature? We can see that it can act most effectually. But in our domestic productions the variability is not directly produced by man, for he can neither originate varieties nor prevent their occurrence. All he can do is to preserve and accumulate such as do occur. Unintentionally he exposes organic beings to new and changing conditions of life, for under domestication, plant and animal organizations become in some degree plastic, and variability ensues. Similar changes, however, do occur in nature. When it is borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other, and to their environment, and consequently what infinitely-varied diversities of structure may be of advantage to each being under altered conditions, can it then be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life should sometimes occur in the course of tens of thousands of generations? If such do occur, can we doubt, when it is remembered that many more individuals are born than can possibly survive, that individuals possessing any advantage, no matter how slight, over their fellows would have the best chance of surviving and of procreating their kind? Any variation, on the other hand, we may feel sure if in the least degree injurious would be rigidly destroyed. This preservation of useful and favorable variations, and the destruction of those that are injurious, is called Natural Selection, or the Survival of the Fittest. Variations neither advantageous nor deleterious would not be affected by Natural Selection, and would be left either a fluctuating element, as seen in certain polymorphic species, or would alternately become fixed, owing to the nature both of the organism and its conditions.