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 egg 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 he evolved, old features may be strervith 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 Weismann 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 gam,s 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 Wal 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 which is observed in the evolution of mammals and in the progress through oviparous monotremes, prematurely bearing marsupials, and various grades of placentals; the growth of parental care, and the frequent of self-preserving to species-maintaining ends; and finally, the rise of sociality from foundations based in organic kin ship, are well-known facts of animal life which suggest the importance of the re productive factor in evolution.