REPBODUCTION 503 REPRODUCTION "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 Reprodruction 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. Importance of Reproduction in Evo- lution.— -As almost every individual life begins in the intimate union of two liv- ing units — the male cell and the ei^g cell — there is in the nature of the organisms beginning an evident possibility of varia- tion. The two cells, and more especially, the nuclei of the two cells, are inter- mingled; and in the vital combination which results new characteristics may be evolved, old features may be strength- ened, peculiarities may be averaged off. On fertilization as a source of variation, emphasis has been laid by Treviranus, Galton, Brooks, and others, while Hat- schek regards the intermingling as an im- portant counteractive of disadvantageous individual peculiarities, and V/eismann finds in it the sole source of transmissi- ble variations in many-celled animals. In the individual life the antithesis be- tween the reproductive and the nutri- tive functions has many expressions, and in terms of this antithesis not a few lines of variations can be rationalized. Thus, the shortening of the axis of the flower seems to be the result of a check imposed on the vegetative system by the reproduction function; thus, the develop- ment of gymnosperm into augiosperm suggests a continuous subordination of the reproductive carpellary leaf; thus, in almost every natural alliance of phanero- gams may be read a contrast between more and less vegetative types, such as is seen within the limits of a single species in the transitions between the leafy kale and the cauliflower. Among animals the antithesis is expressed in different ways — as in the varied degree in which the reproductive individuals of a hydroid colony are differentiated from the nutri- tive members. In considering the evolution of ani- mals great importance is always — and rightly — attached to the self-preserving struggles and endeavors which secure the satisfaction of nutritive needs; but the species maintaining activities of repro- duction have been not less important. Thus, Darwin insisted on the importance of sexual selection as a factor in evolu- tion, and, though the criticisms of WaU lace and others have lessened the co- gency of Darwin's argument, there can be little doubt that courtship has aided in the evolution of the psychical life of animals. Romanes, too, in his insistence on the importance of isolation, recognizes "the reproductive factor in evolution." For by variations in the reproductive system a species may be divided into mutually sterile sets, which, prevented from intercrossing by this physiological barrier, are free to develop along di- vergent paths. The increase of reproductive sacrifice
