80 GRACE P. HAYS 



later generations. This probably points to some unstable 

 factor in the underlying situation. 



At present it seems to be simplest to think of either Poly- 

 dactyly or syndactyly as due to the introduction of some factor 

 not normally present. Local conditions are never identically 

 the same, and the variations in the abnormality may be the 

 result of variations in the local conditions rather than the result 

 of variations in the unusual factor. This idea can be expressed 

 in abstract terms. The individual variable conditions govern- 

 ing the formation of a given extremity may be represented by 

 X. A constant factor, K, may or may not be introduced. If 

 introduced an abnormal condition is produced, .r + K, then, may 

 be said to represent the situation when syndactylous extremities 

 are formed. If, however, the local conditions are sufficiently 

 changed, and if to these changed local conditions the constant 

 factor, K, is added, X -\- K may produce polydactylous extremi- 

 ties. In a given individual the physiological condition of the 

 upper extremities might be represented by X and that of the 

 low^er extremities by x. If, then, the disturbing factor, K, were 

 introduced such an individual would have polydactylous hands 

 and syndactylous feet. 



Such a factor might be either physiological or mechanical. 

 If physiological, it must be a factor which occasionally affects 

 only the upper or lower extremities. Three possible physio- 

 logical factors at once occur to one, infection, malnutrition, and 

 deficient blood supply. In practical effect the last two are the 

 same, for the physiological condition would be the same whether 

 the lack of material was occasioned by an actual lack of the 

 blood supply, or by a lack of the essential constituents in the 

 blood. The evidence taken from cases of rickets makes it clear 

 that there is a distinct connection between nutrition and bone 

 formation (Still, '14, p. 776). If the factor be mechanical it 

 must tend to exert its effect symmetrically. The conception of 

 pressure as the determining factor is not new. The amnion has 

 been thought to be a possible source of pressure, causing both 

 poly- and syndactyly. It has been suggested that Polydactyly 

 may result from pressure of amniotic threads on the distal 



