Struggle for Existence

STRUGGLE FOR EXISTENCE (in biology), a factor introduced by Darwin into his theory of natural selection. By the struggle for existence is understood that contest or competition which constantly goes on between various animals and plants in their infinitely complex relations to other creatures and the physical conditions of life, and that counteraction which organisms offer to unfavorable factors through their organization and behavior. The struggle for existence occupies a central place in Darwin's theory. The latter, as is well known, proceeds from an analysis of the formation of breeds of domestic animals and cultivated plants. Darwin considers the principal factor of this formation to be the principle of "selection," applied consciously or unconsciously by man, who leaves the best forms for breeding or for seeds. The possibility of such selection (called "artificial") lies, according to Darwin, in "variability," i.e., in the fact that children of the same parents or plants originating from seeds of the same capsule are not identical, but differ from each other. The fixation in the offspring of the best qualities of the producers or of plants left for seeds is brought about by virtue of "heredity." Thus, in artificial selection carried out over many generations, the formation of new breeds of domestic animals and cultivated plants depends on the joint action of three factors: variability, heredity, and selecting man. Transferring the principle of selection to nature, where variability and heredity also operate, Darwin came to the thought that under natural conditions, selection performed by man is replaced by the struggle for existence, or competition, which in the struggle for life destroys all organisms that are weak or unadapted to the conditions of existence and preserves the most adapted. Thus, the struggle for existence is, according to Darwin, the cause of natural selection. These three factors—variability, heredity, and the struggle for existence—acting over a series of generations, determine the evolution of organisms toward their most expedient structure. The struggle for existence is considered in the theory of natural selection not so much as a deduction from observations, but as a logical consequence of the "geometric progression" of reproduction in animals and plants, or of the excess of born forms—an idea that occurred to Darwin after his acquaintance with the teachings of Malthus, who applied it to human societies. This term, as Darwin himself pointed out, was understood by him in the broadest sense. This includes "the dependence of some creatures not only on others, but also on all external conditions." The struggle for existence can take various forms: 1. A distinction is made between the struggle between organisms where their stronger constitution triumphs, withstanding various fluctuations in climatic factors—severe heat, cold, desiccation, any harmful properties of air, water, food, soil, etc. A special character of this struggle consists in the fact that its intensity depends not on the number of individuals, but exclusively on the force of the factor's action and the degree of the organism's resistance. If one takes into account the significant fluctuations of climate in different geological epochs, the entire significance of this form of the struggle for existence will be clear, which perhaps destroyed entire groups of the animal and plant kingdoms and brought forward others that were by their constitution more adapted to the new physical conditions. Modern hydrobiology with its very subtle methods for determining the content of particular chemical elements in water has sufficiently shown how close is the dependence of the world of organisms inhabiting the water upon the percentage content of individual elements in it. 2. The second form of the struggle for existence is interspecific struggle and the struggle between varieties of the same species. This struggle can be either an aggressive struggle or a type of competition in the struggle for existence. Aggressive, or direct, struggle amounts to one organism living at the expense of another. Animals feed at the expense of plants, carnivorous animals at the expense of herbivores, parasites at the expense of their hosts. In this case, among the pursuers those triumph which possess better means for pursuit (the parasite possessing better organs of attachment holding it to the host), and among the pursued those that possess better means to avoid pursuit, i.e., among the former—individuals with better sense organs, greater strength, speed, or better organs of attack; among the latter—individuals with better organs of defense, protective coloration, etc. As a result of such struggle, the pursued species is either partially destroyed (only the most adapted individuals survive) or completely destroyed, and then the species pursuing it must switch to other prey (for example, a parasite to a new host), or it is also threatened with destruction. But in this case, a kind of mobile equilibrium is usually established, since the pursuer itself depends on the number of individuals pursued by it. A decrease in prey entails a decrease in the number of pursuers. This decrease gives, in turn, the prey an opportunity to reproduce more strongly, which makes the conditions of existence more favorable for the pursuers, which rapidly increase in number, and so on. This explains why the average number of individuals of a certain species in a given locality usually remains constant and why at the same time "waves of life" are sometimes observed, temporary mass reproductions of certain forms, invasions of mice, mass appearances of harmful pests, etc. It is easy to understand that among the pursued and the pursuers, the most adapted will survive in the struggle for existence. Therefore, one encounters so many complex forms of catching other animals by some (spider webs, carnivorous plants) and defense (mimicry—protective, warning, or intimidating coloration). The struggle for existence between different species takes the form of competition when they are threatened by the same danger or experience the same need. In the forest, there is a struggle for the use of sunlight between different species of trees; on dry soil, different plants struggle with each other for scarce moisture; different species of fish in a closed reservoir may compete for food. In this struggle, another factor plays a huge role—the speed and power of reproduction. Organisms that lay a larger number of eggs, bear a larger number of young, and scatter a larger number of seeds find themselves in a better position. However, the action of this factor is limited by another—the number of destroyed eggs or immature individuals. In general, it can be considered a rule that the greater the number of eggs, seeds, and young produced by organisms, the greater the percentage of them that perish, and the less adapted they are in other respects to the conditions of existence. And conversely, a small number of young (or eggs) is associated either with special parental care for them or with particularly favorable conditions for their reproduction. Thus, albatrosses, which incubate only a single egg each, gather to lay them on isolated oceanic islands, whereas fishes, whose caviar and fry are destroyed in untold numbers, spawn a colossal number of eggs. Thus, in assessing the role of reproduction in the competition of forms, one should take into account the number of individuals reaching sexual maturity. 3. The third form of the struggle for existence is the struggle between individuals of the same species. Here one can also note not very frequent cases of aggressive or direct struggle—such as the struggle of polygamous males with one another during the rutting period, or the eating of smaller males or young by female spiders (cases rarer than commonly thought). Much more frequently, both in animal and plant organisms, indirect struggle or competition is observed in the struggle for identical needs—space, light, nutrition, possession of females, etc., competition that is especially acute at times of sharp changes in external conditions. Thus, it is pointed out that trypanosomes (the causative agents of human sleeping sickness) perish when atoxyl is injected into the patient's blood. However, trypanosomes sometimes reappear after a certain time in the patient, again causing symptoms of the disease. According to Ehrlich, the relapse of the disease is explained by the fact that some trypanosomes, which were more hardy, survived and produced an "atoxyl-resistant race." On the other hand, selection also takes place among humans. Races possessing the greatest immunity (see) in relation to one or another bacterium causing disease are preserved. This indirect struggle begins for the animal from the very moment the egg is formed. Even a slight change in the properties of the egg and its membranes, in its specific gravity, in the place where it is laid, can be of significance for the preservation of the individual's life. A greater or lesser fat content in a fish's egg can either cause the egg to sink to the bottom or make it stay at one or another depth, which entails important consequences: enemies may change quantitatively and qualitatively, the composition of the water, the amount of light, heat, the direction of the current, etc. may also change. A change in the environment at certain stages of development or at certain moments of existence also plays a considerable role in the struggle for existence. Thus, there is a vast number of forms whose larval stages live in water, and adults on land or in the air. With such a division of the individual's life into special periods, the retardation or acceleration of a certain stage of development is of considerable importance in the struggle for existence.

Those forms win in which the duration of the stage during which they are better provided with food and where fewer dangers threaten them increases, and in which the duration of the stage when destruction threatens them decreases. Thus, either the time of embryonic development or, conversely, the time of the sexually mature state can be shortened. In some forms (mayflies) the life of the adult insect is extremely short, limited only to the time necessary for fertilization and for laying eggs. If, however, the onset of sexual maturity is transferred to the larval period, the stage of the adult animal may disappear altogether (neoteny). Thus, the axolotl, the egg-laying tadpole of the ambystoma, may not turn into the ambystoma at all. Finally, the social life of animals, mutual aid between organisms living in societies, or mutual aid between different species (symbiosis), also present significant chances in the struggle for existence. The ability of animals to make various technical structures—burrows, nests, cobwebs, even dams (beavers)—is of no less importance. These two factors—technology and social life—have extremely sharply changed the form of the struggle for existence in humans. With the help of their tools and technology, humans have exterminated or reduced to a minimum many predatory animals, learned to resist the action of unfavorable forces of nature, found a means of fighting the cold in dwellings and clothing, placed a lightning rod against the action of atmospheric electricity, and so on. With the help of the microscope, they discovered the causative agents of diseases and found means (medicine, hygiene, sanitation) to fight the microorganisms most dangerous to themselves, for which the means of the struggle for existence are extraordinary rapidity of reproduction and great endurance. The fight against pests of agriculture and forestry is also one of the important forms of human struggle for food or shelter. As for intraspecific struggle, i.e., the struggle (or competition) of man with man, it has long since left the stage where the population in a given area is determined by the amount of food produced. Therefore, as Pearson showed (based on statistics of human birth and death rates) in the article "Socialism and Evolution," Malthus's doctrine is inapplicable to man. According to statistical data, the majority of the population survives to middle age, i.e., dies after leaving offspring. High infant mortality in the poor classes is covered by their higher birth rate, and fluctuations in the birth rate in humans largely depend on economic and production relations. Thus, Kautsky points to a number of examples of high fertility of human races during their transition to settled agricultural farming. He also points out that the decrease in the number of births, which is manifested in capitalist countries, is associated with a number of special factors: 1) the transition to universal conscription and the growth of large cities, which leads to the development of prostitution and an increase in the number of venereal diseases; 2) the spread of hard female labor; 3) the spread of contraceptives, etc. In a word, from the analysis of the intraspecific struggle for existence in man, it is clear that this struggle, waged over the distribution of goods, is determined only by social and economic factors and has nothing in common with the "biological" struggle for existence. Objections were raised against the significance of the struggle for existence in evolution that its significance has not been proved by definite experiments and observations. But the complexity of the environment in which this struggle takes place, the abundance of intervening factors and causes determining the preservation of species, make the very setup of experiments extremely difficult. This complex dependence in nature of some organisms on others was clear even to Darwin, who gave several classical examples of "chain" dependence (cats, field mice, bumblebees and clover; insectivorous birds feeding on forest insects, forest, cattle gnawing on young trees, flies laying eggs in the navels of newborn animals, parasites of these flies, insectivorous birds feeding on these parasites). Therefore, it is not surprising that darwinists usually refer to the same few observations and experiments that sought to clarify the significance of the struggle for existence for selection. Thus, Bumpus studied 136 sparrows stiffened from a storm; of these, 72 came alive again, and 64 died. Males, females and young were compared separately. The survivors turned out, on average, to be shorter, lighter and possessed a longer humerus than those that died. Weldon, studying crabs near Plymouth for several years, found that crabs change gradually. The width of their carapace becomes narrower and narrower. Weldon attributed this to the action of changed conditions in the bay where the crabs lived. Due to the construction of the dam, much more clay began to settle in the bay. Weldon suggested that the narrow carapace of the crabs protects their gills from silt better than the wide one. Wishing to test his assumption, Weldon placed 250 crabs in vessels with constantly muddy water and examined the width of the carapace; those that died earlier indeed had a wider carapace. Poulton attached dark brown and green mantises to plants of various colors and established that within 17 days those animals that did not match the color of the plant were destroyed by enemies. Green mantises on green plants and brown ones on brown remained intact. The very setup of these experiments, as recently pointed out by M. M. Belyaev, who repeated Poulton's experiments with some improvement in the method (he found that some birds select mantises, while others eat them entirely, no matter what plants they sat on), is usually too primitive. However, Belyaev's experiments are also too fragmentary. In this regard, the work of botanists, especially foresters, for whom the study of the struggle of trees for light in the forest has great practical significance, is of greater value. The enormous importance of the struggle for existence for evolution cannot be denied, but for a more precise clarification of the action of individual factors in various cases, even finer and improved methods are still required.