GENETICS 157 



merely has its ovules fertilised, but its tissues for some distance around 

 are infected and made to take on, in their living, growing substance, 

 some of the quality of the fertilising species. A similar thing occurs, 

 though rarely, when cuttings of one plant are grafted on to another. 

 The living tissue either of graft or of stock, and sometimes of both, is 

 affected by the fusion with it of the tissue of the second plant united 

 with it. And this appears to be a kind of " infection " living particles 

 passing from one to the other and producing a blend of the two kinds 

 of living substance characteristic of each of the united plants. 



Such cases, then, show the enormous symbiogenetic possi- 

 bilities of fertilisation and of crossing, and they may now be 

 seen to support the view here taken of evolution by symbio- 

 genesis, i.e., the gradual and progressive transformation of 

 organic forms on bio-economically pre-determined lines 

 according to mutual, social, or biological values. 



" Now it is quite obvious," continues Dr. Walker, " that 

 the bulk of the characters in any individual are not inherited 

 in the alternative manner. Whether they originated in the 

 remote past from characters that were Mendelian is beside the 

 question; they certainly are not so now." 



I should say, however, that it is of some considerable 

 importance to know whether the bulk of " characters " not now 

 inherited in the alternative manner did in reality originate in 

 the remote past from "characters" that were (mildly) 

 Mendelian ; whether, as I would say, symbiogenesis working 

 through reciprocal differentiation has not at all times been the 

 true fons et origo of all progressive change? Why should we 

 always shun the question of origins? 



Alternative "characters," I submit, imply on the positive 

 side and primarily the preparation and maturation of genuine 

 pairs of reciprocal opposites which normally combine and 

 blend their forces in genuine amphimixis, and given a 

 sufficient super-adequacy of forces jointly evolved in genuine 

 symbiogenesis. (Failing the symbiogenetic explanation, Prof. 

 Dendy seems to me perfectly justified in protesting that it is 

 "a grotesque view " to assume that all species have arisen by 

 " crossing."*) 



* British Association 1914. 



