294 THE THEORY OF THE GENE 



In practice, however, the actual conditions are not 

 always so simple. Many types differ from each other in 

 several genes, each of which has an effect on the same 

 character. Consequently, when they are crossed simple 

 ratios are not found. For example, if a race of corn with a 

 short cob is crossed to a race with a long cob, the next 

 generation has cobs of intermediate length. If these are 

 inbred the following generation has cobs of all sizes. 

 Some are as short as the cobs of one of the original races, 

 others as long as the original long. These stand at the 

 ends. Between them is a series of intermediate sizes. A 

 test of the individuals of this generation shows that there 

 are several pairs of genes that affect the size of the cob. 



Height in man is another such case. A man may be tall 

 because he has long legs, or because he has a long body, 

 or both. Some of the genes may affect all parts, but other 

 genes may affect one region more than another. The 

 result is that the genetic situation is complex and, as yet, 

 not unraveled. Added to this is the probability that the 

 environment may also to some extent affect the end- 

 product. 



These are the multiple factor cases, and students of 

 heredity are trying to determine in each cross how many 

 factors are present. The results are complex only because 

 several or many genes are involved. 



It is this sort of variability that in the earlier days, 

 before Mendel's discovery had been made known, sup- 

 plied natural selection with the evidence on which that 

 theory was based. This question will be considered later, 

 but first must be described the great advance in our 

 understanding of the limitations of the selection theory 

 that was made in 1909 by Johannsen's brilliant work. 



Johannsen carried out his experiments with a garden 

 plant, the princess bean. This bean reproduces exclusively 

 by self-fertilization. As a result of long-continued in- 



