84 THE SHORTER SCIENTIFIC PAPERS 



parental forms, from those having a weight of 3 5-40 

 centigrams there resulted a progeny with an average of 

 57.2 centigrams, while from those having a weight of 65- 

 70 centigrams a progeny was obtained which had an 

 average of 5 5.5 centigrams. In other words, selection had 

 not only failed to make any advancement, but actually 

 resulted in a slight retrogression. Facts quite in accord 

 with this but giving much more pronounced results have 

 been obtained by Tower (1906), Jennings (1908), 

 Johannsen (1909) and others. It should be noted, how- 

 ever, that there have been several experiments, notably 

 those of De Vries with buttercups. Tower with potato 

 beetles, and Smith with Indian corn, where a possible 

 advance of a characer was recorded in a group. Heter- 

 ozygotes here may have been responsible for the result, 

 although again the explanation may consist in the elimi- 

 nation of the effects of a determiner. 



The results in mixed races as exemplified by corn, beans, 

 etc., where selection has gradually improved a group of 

 organisms but finally reached a limit beyond which no 

 progress appeared possible, are comparatively well under- 

 stood and are due, as explained by Shull (1908), to the 

 separation of the pure lines which were present in the race 

 at the beginning. This is where the average agriculturist, 

 horticulturist, and animal breeder has gone far astray and, 

 having succeeded for a few generations in making progress, 

 has failed to understand why he may not continue to be 

 successful. 



Thus we find that attempts to modify a character by 

 selection within pure lines within a small number of gen- 

 eraions have almost universally failed, and that the few 

 apparent results to the contrary must be looked upon with 

 the suspicion that the population was a mixed race and 

 that Mendelian principles applied. 



Once again we are led to propound with still greater 

 emphasis the question, "How then has evolution taken 

 place?" "In what manner have organisms acquired their 

 characters?" "Is it possible to escape the difficulties that 

 confront the investigator on every side?" 



