72 EVOLUTION OF LIVING ORGANISMS. 



then the mean weight of the offspring will be raised, and 

 raised rapidly to the highest possible level that of the 

 strain with the highest mean weight. For since there 

 are more heavy bean-seeds among those belonging to the 

 strains with the higher mean, they will have a better 

 chance of being selected. The offspring of lines with 

 lower mean weight will be gradually eliminated, and at 

 last only those of the strain with the greatest mean 

 weight will remain. Obviously the same result would be 

 reached at once if this strain could be distinguished at 

 the beginning of the experiment, and alone chosen for pro- 

 pagation. Moreover, the effect of selection will come to 

 a stop as soon as this highest point is reached ; unless in 

 the meantime new changes have arisen in the hereditary 

 factors, leading to further possible increase in weight. 

 This limitation is a necessary consequence of the sieve- 

 like action of selection it can only select what is already 

 there. It must also be understood that the modifica- 

 tions due to the environment do not materially alter 

 the results of selection, which acts only by the indirect 

 choice of particular hereditary constitutions. Cumula- 

 tion of results can only take place in so far as new 

 mutations occur in the required direction. 



The same conclusions have been reached on statistical 

 grounds, and may be expressed as follows. Comparing 

 the characters of offspring with those of parents, grand- 

 parents, and great-grandparents, it is found that the 

 resemblance decreases rapidly; so that the "correla- 

 tion " with parent is about ^ or '5, with grandparents '3, 

 with great-grandparents *2, and so on. The "correla- 

 tion " is the ratio expressing the deviation of each genera- 

 tion from the mean of the species. This is the " law of 

 ancestral inheritance" worked out by Galton and modi- 

 fied by Pearson. Pearson has defined it as a rule for 

 predicting the average value of a character in the off- 

 spring from the value of that character in the ancestors. 

 From this point of view, therefore, the contributions to 



