Origin of Species, in 1859, not only laid the foundations of modern biology, but was the dominant impulse of many of the fruitful and important researches which marked the last forty years of the nineteenth century as the great era of scientific progress.

Despite the revolution effected by Darwin, not only in the special sphere of biology, but also in our whole conception of the world, there has been little or no application of Darwinian doctrine to sociology until within quite recent years. It is true that Herbert Spencer had already applied the theory of the survival of the fittest to the life of human societies, before Darwin had applied it to the life of species in general; but while Spencer's conception of the part played by natural law in sociology was luminous and fruitful, the illustrious author of the Synthetic Philosophy made no attempt to give us a synthesis of social history considered from the evolutionist standpoint. Such a synthesis does not yet exist. But, on the Continent, a psychologist of the eminence of M. Ribot has treated the question of heredity under its sociological aspects, and the anthroposociological school associated with the names of Ammon and Vacher de Lapouge has endeavoured to apply Darwinian conceptions systematically to social evolution. Nor can the valuable works of Francis Galton, Ritchie, and Haycraft be overlooked. Although Galton's researches on the heredity of genius and similar problems were primarily undertaken from a purely biological standpoint, their repercussion on the domain of sociology was inevitable.

In the course of this work we shall endeavour to show the essential correlation of the two domains of biology and sociology, as seen not merely in the analogies between the life and development of organisms and the life and development of societies, so brilliantly and conclusively expounded by Spencer; but also in the way in which the law of the survival of the fittest finds application in both. We shall find that selection plays a role in sociology no less important than it plays in biology; we shall examine the

FACTORS OF EVOLUTION

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conditions of heredity, and see how the latest discoveries relating to hereditary transmission are applicable in the field of social evolution; and we shall test the truth of Spencer's dictum that there is no phenomenon in the world of social evolution that has not its root in the conditions of organic life in general.

Variability, heredity, excessive fecundity, and selection, are the four factors which Darwin recognised as the basis of his doctrine of descent. The co-operation of these four factors results in the evolution of living organisms, their progressive differentiation from a single original type and towards an ever higher level.

Variability is, in all probability, a property of every organism. In the whole realm of nature there are no two living beings exactly alike. We are, indeed, unable to penetrate into the world of microscopic organisms deeply enough to appreciate adequately the changes of structure or habit which occur there, but from the examples afforded us in the world of macroorganisms we are justified in concluding that variability is the universal rule. There is no valid reason for supposing that variability is absent in the simplest forms of life, or that its origin is coincident with the development of higher forms. The microorganisms are subjected to the influence of their environment as much as macro-organisms; and no one at all aware of recent researches can doubt that even diminutive creatures such as bacteria are capable of varying greatly.

The distinctive marks by which we recognise and differentiate related species vary as a general rule within well-determined limits. Where it is possible to obtain definite calculations we usually find that fluctuating variations on either side of the mean are by far the most numerous, and those variations which break away from the normal type, either in a regressive or in an ascendant direction, are less numerous. But this is not a universal rule; there are both symmetrical and asymmetrical curves. Artificial breeding especially has disclosed cases like

that of a race of pigeons whose variation from the normal type has diverged to such an extent that the beak is too short and delicate to be able to break open the egg, and the breeder has to assist in the act of hatching.1

In addition to the individual fluctuating variations, there are also sudden and far-reaching transformations which result, by a sudden bound, in a new specific type. The Dutch naturalist De Vries has especially insisted on these sudden transformations to which he has given the name of mutations, and which he distinguishes from the smaller and slower individual variations. According to De Vries, it is to mutation and not to individual variation that the transformation of species has been mainly due.

The second factor of evolution, according to the Darwinian doctrine, is that implied in reproduction itself-the factor of heredity. By heredity is to be understood the faculty possessed by an organism for transmitting its qualities, physical and psychical, to its offspring. All higher organisms possess a peculiar substance localised in the nucleus of the reproductive cell, which transmits the heritable qualities from parent to offspring. Heredity is concerned with those characters which are transmitted directly from the parents to the offspring, or with those qualities which offspring are capable of acquiring independently of their parents, but which, nevertheless, so permeate the transmitting substance that the next generation can inherit them directly. Thus many believe that an individually acquired predisposition to a disease may, as a predisposition, be transmitted to the next generation.

"There is no

The third factor is that of excessive fecundity. exception," wrote Darwin, "to the rule that every organic being naturally increases at so high a rate that, if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty-five years, and

1 Darwin, The Origin of Species, p. 106 (edition 1902).

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at this rate in less than a thousand years there would literally not be standing-room for his progeny. Linnæus has calculated that if an annual plant produced only two seeds-and there is no plant so unproductive as this—and their seedlings next year produce two, and so on, then in twenty years there would be a million plants. The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old. If this be so, after a period of from 740 to 750 years there would be nearly 19,000,000 elephants alive, descended from the first pair."1

Given this enormous fecundity, it is evident that, unless Nature counteracts it by means of an equally enormous elimination, the number of organisms in the world would very soon be too great for the world to contain, for space, air, and food are limited. A definite and constant balance between the rate of multiplication and the rate of elimination must therefore be established. The total number of individuals of a species living in a given space is certainly subject to variation-in certain cases it is extremely variable-but we must, nevertheless, accept the fact that the average number of the members of a species remains constant. Otherwise, if the conditions remain the same, the species would be doomed to speedy annihilation. The number of deaths in a city during one year may be 50,000, in another year 45,000, in the third year 48,000. The total varies, but the average number of the inhabitants maintains itself, and must maintain itself if the city in question is not to disappear. Of course, if the conditions are greatly altered, greatly increased population may become possible and normal.

If we call the average numerical strength of a given species under given conditions the normal number of that species, the

1

Darwin, The Origin of Species, pp. 79, 80 (edition 1902)

same will be determined by the number of progeny brought forth yearly, and the relation of that number to the number of deaths before maturity is reached. The fecundity of a given species being constant, its death-rate must needs be constant also, if the number of its members is, under given conditions, to remain the same. This relation of the death-rate to the fecundity-rate of a species explains why the normal number of that species remains constant. If we know the fecundity-rate of a species, we can calculate its death-rate; for, if the normal number of the species is to remain constant, only two offspring per pair can, on an average, reach maturity and reproduce, for otherwise the constancy of the normal figure of the species would no longer be maintained.

Thus, to take examples which Weismann has given us,1 a pair of storks brings forth annually four offspring, and continues reproducing at the same rate for twenty years, thus begetting a total of eighty offspring. Of these eighty, only two can attain maturity and reproduce, while the other seventy-eight must be destroyed before attaining maturity. A trout lays yearly 600 eggs, and continues doing so during ten years. Out of the total of 6,000 eggs, 5,998 must be destroyed during development and youth, and only two can reach maturity. In other cases the death-rate is far greater. Certain worms bring forth no less than 100,000,000 eggs, of which 99,999,998 must be destroyed before maturity.

Thus a constant ratio is maintained between the fecundity rate and the death-rate of a species, and the maintenance of this ratio is essential if the normal number of that species is to remain constant. It may excite wonder that, given the constancy of this ratio, the fecundity-rate should be so excessively high; and yet, when we come to closer examination, we find that the enormous sum total of births is not superfluous, is not a mere 1 A. Weismann, Vorträge über Deszendenztheorie, i. 39. Jena,