REPRODUCTION. 53 



multiplying asexually, by detaching overgrown portions of 

 themselves which had sufficient vitality to develop into 

 complete forms. But a more economical method is the 

 liberation of special germ cells, in which the qualities of the 

 organism are inherent. This is the primary characteristic 

 of sexual, as opposed to asexual, multiplication. 



It is also conceivable that organisms might have remained 

 approximately like one another in constitution, and at all 

 times very nearly the same, and that they might have 

 liberated similar germ cells capable of immediate develop- 

 ment. Such a race would have illustrated the one charac- 

 teristic of sexual reproduction, the liberation of special germ 

 cells ; but it would have been without that other character- 

 istic of sexual reproduction the amphimixis or fertilisation 

 of dimorphic germ cells, usually produced by different 

 organs in one individual or by distinct male and female 

 individuals. 



Liberation of special germ cells. One must think of 

 this as an economical improvement on the method of start- 

 ing a new life by asexual overgrowth or by the liberation of 

 buds. Asexual reproduction, as Spencer and Haeckel point 

 out, is a mode of growth in which the bud, or whatever it is, 

 becomes distinct or discontinuous from the parent. The 

 buds of a sponge, of a coral, of a sea-mat, or of many 

 Tunicates, remain attached to the parent. If there be a 

 keen struggle for subsistence, this may be disadvantageous ; 

 but in some cases, doubtless, the colonial life which results 

 is a source of strength. In the case of Hydra, however, the 

 buds are set adrift ; the same is true of not a few worms. 

 This liberation of buds takes us nearer the sexual process of 

 liberating special germ cells. But unless the organism is in 

 very favourable nutritive conditions, in which overgrowth is 

 natural, the liberation of buds is an expensive way of 

 continuing the life of a species. Not only so, but we can 

 hardly think of budding even as a possibility in very com- 

 plex organisms, like snails or birds, in which there is much 

 division of labour. Moreover, the peculiarity of true germ 

 cells is that they do not share in building up the " body," 

 and that they retain an organisation continuous in quality 

 with the original germ cell from which the parent arose ; 

 they are thus not very liable to be tainted by the mishaps 



