Physiology of Reproduction. — All growth is, in a certain sense, of the na ture of reproduction. It is an increase in the amount of protoplasm and its at tendant train of substances. Abundance of food material and conditions favorable to rapid assimilation are necessarily ac companied by rapidity of growth; but in the most favoring circumstances there is an inevitable limit to the growth in size of a single cell. It occurs when the rate of assimilation of the constantly increas ing mass of protoplasm becomes equal to the highest possible rate of absorption. Since absorption can only take place through the surfaces, and since, with any given figure of cell, the ratio of volume to surface is a perfectly definite rate as the cell grows, there must be for any given figure of cell a perfectly definite limit of size. For any mass of cells ar ranged in any manner there must be, for similar reasons (though other factors, such as weight, etc., may be operative and varyingly important), a definite limit of size. When in the single-celled animals this limit is reached, or is nearly reached so that starvation begins —and in any case the greater the size of lie cell the less rapid, in proportion to volume, must be the absorption, unless at a certain point other factors at present unknown occur—then division of the cell takes place, by which means, the volume remaining the same, the surface is doubled, so that the ratio of volume to surface and therefore of assimilation to absorption is lowered, and growth is once more possible. This law (first clearly stated by Spencer and by Leuckart) is evidently the expression of a factor con cerned in the initiation of cell division and therefore of the Metazoa, or many celled animals. In the Protozoa, then, reproduction is related to, and in a cer tain sense caused by, a diminution in the possible rate of assimilation, which, to the protoplasm concerned, bears the as pect of an impaired nutrition. In the Metazoa, though reproduction is not so entirely a mere process of cell division as in the Protozoa, a connection between nutrition and reproduction is observable. The common hydra, with an abundant food supply and favoring circumstance, grows rapidly, the growth becoming a process of asexual reproduction and tak ing the form of the production of numer ous buds, which may themselves produce a crop of secondary buds. But if the conditions become less favorable to nu trition through the lessening of the sup ply of food material, then this rapid growth ceases and reproductive organs are formed and sexual reproduction takes place.
Fruit trees are root-pruned in order that the crop of fruit may be abundant; the reason being that, as nutrition is lessened by such pruning, there follows an increase of reproductive activity which takes the form of fruit. If the vegetative activity of the plant be what one desires, then the flower buds are nipped off and sexual activity prevented. A similar result follows from the cas tration of animals. Other factors than the supply of food matter in fluence assimilation and reproduction. As in the case of all molecular movements, variations of temperature are an obvious cause of change of state.
Reproductive maturity—the blossom ing of the individual life—occurs, as has been shown, about the time when growth ceases. In the lower animals sexual maturity is attained relatively sooner than in the higher forms; but there are many strange cases of preco cious and retarded reproduction. Thus
we may contrast our common annuals and the "century plant" or American aloe, or some midges, worms, and even a couple of amphibians, which are repro ductive during larval life, with highly evolved animals, such as the elephants.
But, while reproduction is a blossom ing of the individual life, it is also in a sense the beginning of death. The flower and fruit often end the life of the plant. It may be that the processes of rupture by which some of the simplest organ isms reduce their bulk and multiply their kind are but a few steps from the more diffuse dissolution of death. It is a fact that in some simple animals—e. g. some "worms"—the parent, and especially the mother, ruptures and dies in liberating the reproductive elements. So, among higher forms, not a few insects—may flies, locusts, butterflies—die a few hours after reproduction. The exhaustion is fatal, and the males are sometimes vic tims as well as their mates. In higher organisms the fatality of the reproduc tive sacrifice has been greatly lessened, yet death may tragically occur, even in human life, as the direct nemesis of re production. In short, the process by which new lives begin, by which the con tinued life of the species is secured, tends to be antagonistic to the life of the parent individuals. The old leaves fall off the tree, and their places are filled by others.
Rate of Reproduction and Increase.— The rate of reproduction depends on the constitution of the individual organism and on its immediate environment and nutrition. The rate of increase, which is much more difficult to estimate, de pends on the wide and complex conditions of life which are often included in the phrase "the struggle for existence." While it is true that organisms some times exhibit an extraordinary increase in numbers in favorable areas and sea sons, and while we know of many forms and even of whole races which have dwindled away and become extinct, the fluctuations in the numbers of plants and animals seem for the most part to be im perceptibly gradual. Their rate of re production is adjusted to the conditions of their life; the rise or fall of the popu lation is seldom emphatic. The essay of Malthus (1798), in which he showed that the increase of human population tended to outrun the means of subsistence, but was met by various checks, afforded sug gestions to Darwin and Wallace, who ex tended the induction of Malthus to plants and animals, recognizing in their increase the fundamental condition of the struggle for existence, and analyzing the checks as various forms of natural selection. But Herbert Spencer's analy sis of the laws of multiplication was even more penetrating. Including under the term individuation all those race preservative processes by which individ ual life is completed and maintained, and under the term genesis all those processes aiding the formation and per fecting of new individuals, he showed both inductively and deductively that in dividuation and genesis vary inversely. Genesis decreases as individuation in creases, but not quite so fast; in other words, progressive evolution in the direction of individuation is asso ciated with a diminishing rate of re production.