DEFINITE DIFFERENCES 77 



the species A and C have each a characteristic mean 

 size of fruit, and the existence of this definite mean 

 is not affected by the fact that the range of variation 



FIG 2. 



overlaps in all three cases. Species B, on the other 

 hand, seems to show signs of division into at least two 

 separate groups. 



Differences of a similar kind are sometimes to be 

 found among the progeny of the same individuals. 

 Races of garden peas may be selected which, amongst 

 other differences, are characterized by the presence of 

 large and of small seeds respectively. In each case 

 there is variation of a normal kind about a mean value, 

 but in each case the mean is quite distinct. There is 

 evidence that if a race of large-seeded peas is crossed 

 with a small-seeded variety, and the resulting cross-bred 

 plants are self-fertilized, their progeny in the second 

 generation will be separable into different groups, and 

 some of these will show almost exactly the same size 



