KEPRODUCTION 502 REPRODXTCTION usually exhibited by comparatively simple and by sedentary animals, and seems indeed to be natural to vegetative organisms. Budding is only possible when the organism is not very highly dif- ferentiated, or when part of the body re- tains many indifferent units; moreover, it is an expensive way of securing the continuance of generation, and is without the advantage to the species which un- doubtedly results from the mingling of two life-currents in sexual reproducton. Sexual reproduction in its fully dif- ferentiated form involves (a) the dis- tinctness of two parent organisms, (b) the formation of two different kinds of i-eproductive elements — e. g. spermato- zoa produced by the male and ova by the female, and (c) the fertilization of the egg cell by a male element. Moreover the process of sexual reproduction also includes the sexual union of the two parents, or other ways in which fertili- zation is secured, while in some cases the fertilized ovum develops in organic re- lation with the mother organism, from which it is eventually separated as an embryo. 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
- he 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 O'f 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 somie simple animals — e. g. some
