160 MENDELISM chap. 



size of the crop will effect a considerable difference in the 

 profit. It is the general experience of seedsmen and 

 others that differences of this nature are often capable 

 of being developed up to a certain point by a process of 

 careful selection each generation. At first sight this ap- 

 pears to be something very like the gradual accumulation 

 of minute variations through the continuous application 

 of a selective process. Some recent experiments by Pro- 

 fessor Johannsen of Copenhagen set the matter in a differ- 

 ent light. One of his investigations deals with the in- 

 heritance of the weight of beans, but as an account of 

 these experiments would involve us in the consideration of 

 a large amount of detail we may take a simple imaginary 

 case to illustrate the nature of the conclusions at which he 

 arrived. If we weigh a number of seeds collected from a 

 patch of plants such as Johannsen's beans we should find 

 that they varied considerably in size. The majority 

 would probably not diverge very greatly from the general 

 average, and as we approached the high or low extreme 

 we should find a constantly decreasing number of in- 

 dividuals with these weights. Let us suppose that the 

 weight of our seed varied between 4 and 20 grains, that 

 the greatest number of seeds were of the mean weight, viz. 

 12 grains, and that as we passed to either extreme at 4 

 and 20 the number became regularly less. The weight 

 relation of such a collection of seeds can be expressed by 

 the accompanying curve (Fig. 30). Now if we select for 



