222 GENERAL BIOLOGY 



of a quantity of beans and plotted the variation 

 curve for the lot. He found that it followed the 

 normal curve, i.e. the variations were of the fluctuat- 

 ing sort, grouping themselves about a mean. He 

 then selected beans from both extremes for planting, 

 and saw to it that the flowers on the resultant plants 

 were self -fertilized. As might be expected, the 

 progeny " took after " the parents ; the beans of 

 the second generation that were descended from the 

 lightest beans were all of less weight, whereas those 

 from the heavier were all heavier than the average. 

 The variation curves for the progeny of each bean 

 were also of the normal type, but with a much 

 narrower class range than the curve for a miscel- 

 laneous lot of beans taken at random. From 

 these smaller groups, each the progeny of a single 

 seed, extreme variates were selected and planted. 

 But the progeny of these seeds, when weighed, were 

 found to group themselves in a curve with essentially 

 the same mode as that of their progenitors. That 

 is, within the group of plants descended from a 

 common (self-fertilized) ancestor, selection has no 

 influence, one way or the other, in shifting the mode 

 for the next generation. Johanssen called these 

 smaller groups within the mass of the species pure 

 lines, or genotypes, in contrast to the larger group 

 which all together they go to make up, and which 

 he called the phsenotype. Ordinarily, cross-fertili- 

 zation is continually mixing one pure line with 

 another, and as the greater number of such pure lines 

 group themselves about the mean of the whole 



