278 APPENDIX D 



are enough to stamp Mendel's discovery as among 

 the greatest in the history of the biological sciences. 

 But it does not alter the Darwin-Wallace concep- 

 tion of evolution in nature. 



The pattern of each mimetic form of the poly- 

 morphic female of Papilio dardanus is a complex 

 unit character as defined by Castle, yet all of 

 them exhibit clear evidence of a past history of 

 'continuous' improvement in the likeness to 

 their respective models. 



Sports such as those which arise by the dropping 

 out of some definite element and the consequent 

 sudden change to white of the whole or a part 

 of the pigment of an animal or flower, are a type 

 of the appearances which are attractive and 

 interesting to man, and have become subject 

 to artificial selection. And it is with material 

 thus derived that nearly the whole of Mendelian 

 research has been hitherto concerned. Selection 

 may occasionally operate along similar lines in 

 nature, as when an animal migrates into some 

 snow-covered area, but no one who has 'reflected 

 much upon the struggle for existence can believe 

 that it is the usual method of evolution. 



Similarly with regard to the limited advance 

 that is possible when fluctuating variability is 

 artificially selected. Man is able, in a few genera- 

 tions, to double the percentage of sugar produced 

 by the beet. By selecting for this quality alone, 

 he profoundly modifies the relationship of one 

 particular function to the plant as a whole, and 



