CHAPTER II. 



POLYDACTYLISM. 



The possession of extra toes is a character that crops out again and 

 again among the higher, typically 5-toed vertebrates. Many cases have 

 been cited in works on human and mammahan teratology (c/. Bateson, 

 1904, and Schwalbe, 1906), and it is recognized that this abnormality is very 

 strongly inherited in man. Bateson and Saunders, and Punnett (1902 and 

 1905), Hurst (1905), and Barfurth (1908), as well as myself in my earher 

 report, have demonstrated the inheritableness of the character in poultry. 

 Bateson and Punnett (1905, p. 114) say: "The normal foot, though 

 commonly recessive, may sometimes dominate over the extra-toe character, 

 and this heterozygote may give equahty when bred with recessives, just 

 as if it were an ordinary DR." Altogether, the inheritance of extra-toe 

 diverges so far from typical Mendelian results as to deserve further study. 



A. TYPES OF POLYDACTYLISM. 



There are two main types of polydactylism : that in which the inner toe 

 (I) of the normal foot is replaced by 2 simple toes, and that in which it is 

 replaced by two toes, of which the mediad is simple and the laterad is 

 divided distally. The former tjq^e is characteristic of the Houdans ; 

 the latter is usually associated with the Silkies. Both conditions are, how- 

 ever, found in both races. The simplest condition is seen in manj^ Houdans 

 of my strain. It consists of 2 equal, medium-sized toes (F and I") lying 

 close together and parallel to or slightly convex towards each other. This 

 condition indicates that the 2 toes, together, are to be regarded as the 

 equivalent of the normal single toe occupying the same position. The 2 

 toes are, I conjecture, derived from the single toe by splitting. The first 

 series of changes consists of the increase in length of the lateral element 

 (P') and a corresponding decrease of the median element (P). In the last 

 term of the series there are only 4 toes on the foot, but the inner toe is not 

 like the normal inner toe of poultry, but is a much elongated I". 



In the Silkie, also, the series begins with 2 small, closely-appUed toes 

 (P and I") . But when there are only 2 toes the lateral one is usually much 

 the larger. T5^ically this lateral toe is, as stated, split, so that the nail is 

 double, and the degree of splitting is variable, in extreme cases involving 

 half or more than half of the toe. A second series of changes consists of 

 the gradual reduction of toe I' (often concomitantly with an increase in I") 

 which may end in its entire disappearance and thus reduce the number of 

 toes to 5, but these are not equivalent to the 5 toes of the Houdans, since 



2 17 



