xxvi Introduction. 



In the sexual ^gg, however, the second division follows 

 immediately upon the first, and the extrusion of one 

 oozoon reduces the chromosomes by half their number, 

 which is again made good by those of one spermatozoon. 

 While, therefore, in the parthenogenetic egg those 

 ' material bearers of ancestral qualities ' remain the 

 same from generation to generation, they are changed 

 in the sexual generation by each cross fertilization, and 

 in this exists the advantage of amphimixis. For the 

 evolution of species, individual variation is essential, 

 and those individuals who derive half their germ-plasm 

 from one parent, and half from another, each bringing 

 in different lines of ancestors, must present greater 

 variation than those who derive their germ-plasm from 

 one parent and one ancestral line. Sexual reproduction 

 is thus of advantage to the species in so far as it helps 

 to increase inherent tendency to variation, and enables 

 evolution to take place by the steady accumulation of 

 slight beneficial differences \ The random withdrawal of 

 the ids of ancestors by a species of lot, as laid down by 

 Weismann, must be considered simply as a provisional 

 h3^othesis ; and it seems, according to Prof Hartog, 

 that mathematically its effect, if it took place at all, 

 would be the opposite of what Weismann expected '^ 



Although the polar bodies were first noticed by 

 Miiller in 1848, their importance has only been recently 

 appreciated. Balfour^ considered their extrusion as 

 the removal of part of the constituents of the ger- 

 minal vesicle which are necessary for its functions as 

 a complete and independent nucleus to make room for 

 equivalent parts of the spermatic nucleus ; and he con- 



^ Darwin, Origin of Species, p. 132. 



2 Prof. Hartog, Nature, vols, xliv and xlv, 1893. 



^ F. M. Balfour, Comparative Embryology, vol. i. p. 63, 1880. 



