250 THE EVOLUTION THEORY 



lay partlienogenetic eggs {E'^ and E'^), and from these there emerge, 

 late in autumn, the members of the single bi -sexual generation, 

 males and females {F^ and F^), l)oth very minute and wingless, 

 without a piercing proboscis, and thus incapable of taking food. 

 These pair, and the female lays a single egg (A) under the bark of the 

 vine, from which the leaves are now falling ; this egg survives the 

 winter, and from it in the following April or May there emerges once 

 more a partlienogenetic female. 



It could hardly be more plainly shown than it is bj^ this case 

 that the importance of amphimixis is something quite apart from 

 reproduction and multiplication, for here the number of individuals is 

 not only not increased by amphimixis, but is materially diminished, 

 being indeed lessened by a half. By the retention of amphimixis, 

 the species gains in this case no advantage except the mingling of two 

 germ- plasms. 



Something similar occurs in plants which exhibit alternation of 

 generations, for instance the ferns, in which the sexual generation, the 

 so-called prothallium or prothallus, contributes nothing to the multi- 

 'plication of the plant, since only a single egg-cell is developed ; and the 

 same is true of the mosses. In both cases multiplication depends 

 solely on the asexual generation, which, as the so-called ' moss-fruit ' or 

 ' fern-plant proper,' produces an enormous number of spores, in addition 

 to multiplying by runners. 



To sum up : we have seen that self-fertilization does occur in 

 hermaphrodite animals, where otherwise the species would be in 

 danger of extinction, but this is never the bole and exclusive mode of 

 fertilization ^, for hermaphrodite species have always the possibility 

 of securing inter-crossing of individuals, and that in various ways, 

 whether by the intervention of ' primordial males ' or by an occasional 

 or a periodic alternation of self-fertilization and mutual fertilization. 

 Pure parthenogenesis enduring through innumerable generations does 

 ^ appear to occur, but in most cases unisexual generations alternate with 

 bi-sexual, so that a stereotyping of the germ-plasm with complete 

 uniformity of ids is obviated. 



We must now briefly consider the higher plants with reference to 

 the maintenance of diversity in the germ-plasm through crossing. 



We saw in an earlier lecture that most flowers are hermaphrodite, 

 but that ih&y do not fertilize themselves, and are adapted for crossing, 



* As to the cases Maupas has brought into notice, of permanent and apparently 

 exclusive self-fertilization in Ehabditidai (round worms), it seems fair to say that they 

 have not been as yet sufficiently investigated to admit of a secure appreciation of their 

 value in their theoretical bearings, Cf. Arch. Zool. Exper., 3rd ser., vol. viii, 1900. 



