CHAPTER III. 



SYNDACTYLISM. 



A. STATEMENT OF PROBLEM. 



In man, various mammals, and some birds two or more adjacent 

 fingers are sometimes intimately connected by an extension of the web 

 that is normally a mere rudiment at their base. Such a condition is known 

 as syndactylism. A good introductory account of syndactylism is given 

 b}^ Bateson (1904, pp. 356-358). Taking a number of cases of sjmdactylism 

 together, he says: "A progressive series may be arranged showing every 

 condition, beginning from an imperfect webbing together of the proximal 

 phalanges to the state in which two digits are intimately united even in their 

 bones, and perhaps even to the condition in which two digits are represented 

 by a single digit." He also calls attention to the fact that in the human 

 hand " there is a considerable preponderance of cases of union between the 

 digits III and iv;" while in the foot the united digits "are nearly always 

 II and III." The matter of syndactylism in birds has a pecuUar interest 

 because of the fact that among wading and swimming birds syndactyhsm 

 has become a normal condition of the feet, and, moreover, just this feature 

 is one that has become classical in evolutionary history, because Lamarck 

 thought it well illustrated his idea of the origin of an organ by effort and use. 



Concerning the cause of syndactyhsm Uttle can be said. Both in mam- 

 mals and birds the digits are indicated before they are freed from lateral 

 tissue connections. The Unear development of the fingers is in part accom- 

 panied by a cutting back of this primordial web, in part by a growth 

 beyond it. In syndactylism growth of the web keeps pace with that of the 

 fingers. From this point of view syndactylism may be regarded as due to 

 a peculiar excessive development of the web.* In some human cases 

 adhesions of the apex of the appendage to the embryonic membranes has 

 stimulated the growth of the interdigital membrane, resulting in syndactyl- 

 ism. But it would be absurd to attempt to explain syndactylism in general 

 on this ground. The more "normal" forms of syndactyhsm, as seen in 

 poultry, still want for a causal explanation. 



jNIost of the cases of syndactylism whose inheritance is about to be 

 described arose in a single strain of fowl and can, indeed, be traced back to 

 a single bird. This ancestor is No. 121, a Dark Brahma hen described in a 

 previous report. f It was only in the search for the origin of the exaggerated 

 forms of syndactylism observed in some of her descendants that an unusu- 



Lewis and Embleton (1908, p. 45) present strong arguments against the theory that eyndactylism is due to 

 arrested development. 



t Davenport, 1906, page 34. Plate V 



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