IXBKEEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION 259 



and asexual o-euerations alternate with each other, so that ' alternation 

 of generations ' occurs, as is common in lo\Yer animals, especially 

 polyps, medusa^, and worms. 



But it sometimes happens among plants that the sexual repro- 

 duction is absent, and that a species reproduces by the asexual mode 

 only, and this is the case which we must now consider more closel}'. 



Let us first of all seek to gain clearness as to the composition of 

 the germ-plasm in the case of purely asexual multiplication, and 

 what conclusions may be drawn from this, and then let us compare 

 these with the known observational data, and it will be apparent that in 

 individuals which have arisen by budding the complete gerrn-plasm 

 of the species must be contained ; the number of ids will not only 

 remain the same in the bud as it was in the mother plant, but the 

 number of different ids will not be diminished. The case is analogous 

 to that of pure parthenogenesis, in which the absence of the second 

 maturation-division of the ovum allows the germ-plasm to retain 

 the full complement of ids. Charles Darwin held that purely asexual 

 multiplication was 'closely analogous to long-continued self-fertilization,' 

 yet, as we have seen, according to our theory there must be a not 

 inconsiderable difference between the two processes, depending on the 

 fact that in exclusive self-fertilization the number of different ids is 

 continually decreasing, while in purely asexual reproduction the 

 germ-plasm loses nothing of the diversity of its ids. If, therefore, the 

 germ-plasm in purely asexual reproduction no longer receives fresh 

 ids through amphimixis, it at least loses none of those it formerly 

 possessed. Although we cannot consider it adapted for entering upon 

 new adaptations in many directions, yet we may expect that the 

 species will continue to reproduce unchanged for longer than in the 

 case of exclusive self-fertilization, the more so since all unfavourable 

 variational tendencies which crop up are eliminated as soon as they 

 attain to selection-value, and, as in the case of parthenogenesis, they 

 are eliminated without being mingled with other lines of descent. 



Let us take, for instance, the purely asexual reproduction which 

 obtains in Alga^ of the genus Lainiinaria, in regard to which it is 

 stated that it multiplies only through asexual swarm-spores. There 

 are quite a number of species of this large tangle, and if it should be 

 established that in all these the spore-cells really do not conjugate, 

 then the case would prove that the species of a genus can maintain 

 a well-defined existence for a long time after amphimixis has been 

 given up. But this Avould not be a proof of the possibility of speciet>- 

 fovTnation, for that the ancestral forms of the Laminarians must have 

 possessed amphigony may be assumed, since their nearest relatives 



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