VARIATION AND MUTATION 



14: 



Fig. 86.— Child with six 

 toes on each foot. (After 

 Bateson.) 



Species form according to the law of error. Although often not large, 

 they are yet rarely so minute as those differences which distinguish the 

 adjacent members in any series of individuals arranged on a basis of 

 continuous or fluctuating variation. Mutations, according to the usage 

 of de Vries, discontinuous variations may or may not be. Thus, all 

 mutations might be called discontinuous 

 variations, although not all discontinuous 

 variations are necessarily de Vriesian muta- 

 tions, that is, certain to breed true under 

 varying conditions of environment. 



"Asa matter of fact, not all continuous 

 variation follows the law of error : the curve 

 or polygon of frequency is not infrequently 

 an unsymmetrical one: 'skewness' prevails; 

 that is, the highest part of the curve may be 



nearer one end, or the curve may even be bimodal. But neverthe- 

 less the 'continuity' of the variations is unmistakable. In a suffi- 

 ciently large series the extremes of the range are perfectly connected 

 with the mode or modes and hence with each other by gradatory steps 

 very small in size. Whatever the largeness of the difference between 

 the extremes, any two adjacent members of the series are hardly distin- 

 guishable. This gradual 

 kind of variation, in- 

 sensible, but yet effective 

 (as regards widely se])a- 

 rated members of the 

 series), is most typically 

 illustrated in cases of 

 what Bateson calls 'sub- 

 stantive' variation, that 

 is, where the varying 

 characteristic is one of 

 pattern, of length, width, 

 or bulk, of the curving 

 of a vein or leg or sj)ine. 

 Excellent examj)les of this 

 continuous substantive variation are presented by the abdominal and 

 face patterns of Vesjm (see Fig. 76), and the clytral i)attern of Din- 

 brotica (see Fig. 75). 



"According to Bateson, variations in number of antennal and 

 tarsal segments, number of spines, hairs, or other processes, and other 



Fig. 87. — Eyestalks of a decapod dissected out: 

 on the right an antenna has regenerated out in 

 place of an amputated eye; opt., optic nerve. 

 (After Herbst.) 



