274 THE AMERICAN NATURALIST [VOL. XLIII 



a race of pure scarlet poppies was finally obtained with 

 no indication of their yellow ancestor. This case is cited 

 by De Vries as one of mutation, but certainly it required 

 more than one mutation to bring about the result. 1 



Discontinuity may often be more apparent than real, 

 the discontinuous variations in the soma being the out- 

 come of continuous variability instead of abrupt changes 

 in the germ. Let us consider from this point of view 

 the occurrence of digital anomalies such as polydactylism, 

 cleft hand, etc., which are frequently cited as illustrations 

 of discontinuous variability. These anomalies are often 

 strongly inherited, but in most cases which have been 

 fully studied, the inheritance, while partly alternative, 

 is not Mendelian. In the race of polydactylous guinea 

 pigs which Castle has produced and bred for a number 

 of generations the anomaly appeared in different indi- 

 viduals in various stages of completeness. The parent 

 of the group, a male, bore an imperfectly developed toe 

 on his left hind foot. The extra toe contained a claw and 

 probably the phalanges, but it was loosely attached and 

 hung limply down on one side. This male produced 27 

 young, of which 15 were polydactylous. Of the latter some 

 had an extra digit on both hind feet, others had it on but 

 one, and in a few individuals the digit was more fully 

 developed than in the father. In subsequent generations 

 the anomaly appeared in very different degrees of de- 

 velopment, some animals having a fully developed digit, 

 others having a loosely hanging toe with or without a nail, 

 while in extreme cases there was only a fleshy bag of skin 

 without bones or claw which often shriveled up and dis- 

 appeared a few days after birth. The variation, when 

 appearing on one side alone more frequently was limited 

 to the left side (1. 630, r. 589), and when unequal on the 

 two sides the left one was usually the better developed. 

 Normal and polydactylous individuals did not segregate 

 in Mendelian ratios. In some instances the normal condi- 



a See also the experiments of MacCurdy and Castle in relation of con- 

 tinuous and discontinuous variation in rats. Publications of the Carnegie 

 Institution, No. 70, 3907. 



