Fig. 77. Pedigree of boys shown in Fig. 76. (After Blakeslee.)
Darwin knew that if selection of particular kinds of individuals of a population takes place the next generation is affected. If the taller men of a community are selected the average of their offspring will be taller than the average of the former population. If selection for tallness again takes place, still taller men will on the average arise. If, amongst these, selection again makes a choice the process would, he thought, continue (fig. 79).
Fig. 78. A race of corn reared under different conditions.
We now recognize that this statement contains an important truth, but we have found that it contains only a part of the truth. Any one who repeats for himself this kind of selection experiment will find that while his average class will often change in the direction of his selection, the process slows down as a rule rather suddenly (fig. 80). He finds, moreover, that the limits of variability are not necessarily transcended as the process continues even although the average may for a while be increased. More tall men may be produced by selection of this kind, but the tallest men are not necessarily any taller than the tallest in the original population.
