\i 



88 The Third and Fourth Generation 



and turned-in soles of the feet facilitate 

 climbing. 



So many cases similar to that of the peas 

 mentioned above have been found that it is 

 now recognized as unusual to find a character 

 dependent on only one factor. So the factorial 

 hypothesis has come into vogue: this means 

 that characters as such are not heritable things 

 but that there are certain factors that are really 

 the heritable units and their interplay causes 

 the appearance of a particular character. The 

 manifest character in the case given is the 

 purple color of the blossom. What the factors 

 really are we do not know, but we do believe 

 they are heritable units and that together 

 they produce the visible color. Occasionally 

 one factor may be the sole cause of the 

 character; then, of course, the latter acts as 

 the heritable unit. Usually a character is 

 the resultant of the interplay of several fac- 

 tors. Sometimes • one factor is instrumental 

 in determining several characters. At times 

 two or more factors may independently pro- 

 duce like results. Some concrete illustrations 



