SYNDACTYLISM. 



35 



the numbers lag behind, in the higher proportions 40 to 25 per cent. So 

 we reach the conclusion that, as in polydactyhsm, so in syndactyUsm 

 dominance is very imperfect. But there is this difference, that in syndactyl- 

 ism dominance is so imperfect that the dominant condition rarely shows 

 itseh in heterozygotes and even fails in many pure dominants. The strik- 

 ing fact, the one that assures us the segregation is nevertheless occurring 

 in this case too, is that some families (whose two parents are extracted 

 recessives) throw 100 per cent recessives. 



Table 25. 



These studies on syndactyhsm in poultry may be used for a critical 

 examination of the recent work of Lewis and Embleton (1908) on syndactyl- 

 ism in man. The cases described by them follow the types I have just 

 described in poultry. Their fig. 18 corresponds to my types a and a ; figs. 

 10 and 11 to my type /?. The "crossbones" referred to by the authors 

 correspond to bones of the "curved toe." The facts presented by the 

 authors support the idea that syndactyhsm is dominant rather than reces- 

 sive, but they deny the appUcation of Mendelian principles to this case. 

 Actually, the foot deformities described by Lewis and Embleton are inher- 

 ited much hke syndactyhsm in poultry. No extracted normal (recessive) 

 extremity produces the abnormal condition. Heterozygotes show much 

 variation, from very abnormal to slightly abnormal (possibly perfectly nor- 

 mal?) appendages. Dominance is, indeed, much more potent than in poultry. 



The authors' denial of the application of Mendelism to this case seems 

 to be based on an all too superficial consideration of the hereditary behavior 

 of the character and a tendency to "mass" statistics — a procedure that 

 tends to obscure the interpretation of the data of heredity. 



As to the inheritance of type, my statistics are not extensive enough to 

 give a final answer, but if all types be grouped into those with straight and 

 those with curved toes, then in crosses of straight-toed syndactyl and nor- 

 mal 33 per cent of the offspring were of the curved type, whereas in crosses 

 of curved-toed syndactyls and normal 45 per cent were of the curved type. 

 These averages depend on 22 and 15 individuals, respectively. They lead us 

 to look for an inheritance of type when more extensive data shall be available. 



Syndactylism is a typical sport, that is, a rather large mutation having 

 a teratological aspect. The question arises. Does it prove to be prejudicial 



