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,x—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 Aydra, 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 over- 
growth 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 
complex 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 
