XII.] CONJUGATION AND SEXUAL REPRODUCTION. 221 



The means by which this limitation is exercised are different 

 in the two classes, not by any means because parthenogenesis 

 could not have been introduced among the lower Metazoa, 

 but because nature did not require it, but resorted to the far 

 more practicable and flexible methods of fission and budding. 

 When these ceased to be available, she was compelled to 

 alter the sexual cells in such a way that their powers of 

 development were no longer connected with amphimixis. 



There are indeed no plants wholly devoid of the power of 

 reproduction by buds. Not only the formation of stocks but 

 also the copious increase of persons and stocks^ by means 

 of buds is everywhere at the disposal of nature, and she has 

 made a lavish use of them. With this is probably connected the 

 fact that parthenogenesis is unusually rare in plants and only 

 occurs in a few groups. I must leave it to abler botanists to 

 investigate the grounds upon which unicellular germs, originallj- 

 adapted for amphimixis, have been, in certain exceptional cases, 

 afterwards transformed into parthenogenetic germs. The alter- 

 nation of generation, so prevalent among the lower classes 

 of plants, takes a form somewhat different from that found 

 in the lower groups of animals, inasmuch as, not only the 

 multiplication which is associated with amphimixis, but also 

 that which is without, viz. agamic, depends upon unicellular 

 germs. Ferns, Mosses, and Lycopodiums produce vast quanti- 

 ties of spores, the unicellular nature of which certainly does 

 not follow from any former connexion with amphimixis in 

 remote ancestors. It is far more probable that the unicellular 

 condition has proved necessary in order to confer other advan- 

 tages which, as has been suggested above, depend upon a minute 

 size :— the lightness which facilitates transport by wind and 

 water, and the possibility of production in enormous numbers. 



In conclusion, it has been shown that amphimixis is every- 

 where present among the vital phenomena of a species when 

 its existence is without injury to other vital interests, — that 

 it appears, in the Protozoa, independently of reproduction, 

 when a connexion with the latter was possible but unneces- 

 sary,— and that, in the Metazoa, it is bound up with reproduction, 

 inasmuch as its existence only thus became possible. It has 



^ For a definition of this term see page 213. 



