xxxii Introduction. 



this way we have a means not only of securing variation, 

 but of maintaining the fixity of species, which is equally 

 important. 



Alternating generations disappear as we ascend higher 

 in the animal and vegetable scale, or as life lengthens 

 beyond the period when seasonal alternation could be 

 of advantage; then the purpose in view seems to be the 

 approximation and assimilation of consecutive genera- 

 tions, and the continuous uniformity of the species. 

 It is not quite clear how this result is attained, but the 

 polar bodies cease to monopolize the transmission of 

 the unalterable germ-plasm; probably another nuclear 

 division, after fertilization and before ontogeny has 

 begun, is added to them \ and well marked atavism 

 is only found as a pathological occurrence when the 

 assimilating forces fail. 



It is next of interest to inquire how the various 

 structures of the gall came to be evolved. It may be 

 taken as perfectly certain that the tree does not form 

 them in a disinterested manner for the sake of the 

 Cynips. Darwin says : ^ If it could be proved that any 

 part of the structure of any one species had been formed 

 for the exclusive good of another species, it would 

 annihilate my theory, for such could not have been 

 produced through natural selection -.' So far as galls 

 are concerned, Darwin's theory is perfectly safe. The 

 'excitatory emanations,' as Professor Romanes^ aptly 

 calls them, which lead to gall-growth, can only have 

 arisen by gradual and increasing improvements in the 

 initial stages of their formation, acting through natural 

 selection, over an unlimited period of time, and through 

 numerous consecutive species 



^ Weismann. Germ-plasm, p. 192. ^ Darwin, Origin of Species, 



chap. vi. " Romanes, Nature, vol. xli. p. 80, 1889. 



