48 REPRODUCriON AND LIFE-HISTORY OF ANIMALS. 



organism are inherent. This is the primary characteristic 

 of sexual, as opposed to asexual multiplication. 



It is also an evident possibility that the organisms of 

 a species might have remained approximately like one 

 another in constitution, and at all times very nearly the 

 same. Then there would have been no difference of sex, no 

 males and females, nor even hermaphrodites with alternating 

 male and female periods. It is quite possible to think of a 

 race of animals in which all the individuals were approx- 

 imately the same, and liberated similar germ-cells ^yhich 

 grew up right away. This would be sexual as distinguished 

 from asexual multiplication, but it would not exhibit that 

 other characteristic of sexual reproduction, — the existence of 

 dimorphic germ-cells, which come to nothing if they do not 

 combine, and of different kinds of individuals (male and 

 female), or of different sexual ])eriods in the life of one 

 (hermaphrodite) individual. 



The Liberation of Special Germ-Cells. — I believe that one 

 must think of this as an economical improvement on the 

 method of starting a new life by asexual overgrowth or by the 

 liberation of buds. Asexual reproduction, as Spencer and 

 Hjeckel have shown, is a mode of overgrowth in which the 

 bud, or whatever it is, may become discontinuous from the 

 parent. The buds of a sponge, of a coral, of a sea-mat 

 (Polyzoon), or of many Tunicatcs, remain attached to the 

 parent. If there be a keen struggle for subsistence, this 

 may be very disadvantageous; but in some cases, doubt- 

 less, 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 or§;anism is in very favour- 

 able nutritive conditions, in which overgrowth is natural, 

 the liberation of buds is evidently 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 a 

 true germ-cell is, that it is unspecialised, continuous in quality 

 with the original germ-cell from which the parent arose, and 

 not very liable to be tainted by any mishaps which may 



