430 THE GERM-PLAiiM 



The sudden appearance of polydactylism and its great ten- 

 dency to transmissibility can, moreover, be easily understood if 

 we suppose that excessive local nutrition has caused the group 

 of determinants in question to become doubled.* For when 

 this doubling has once occurred in several ids in the germ-plasm, 

 it must be capable of being transmitted on account of the con- 

 tinuity of the germ-plasm; and the degree of certainty with 

 which this will take place will increase as the number of ids in 

 which it has occurred becomes greater. I am entirely of Ernst 

 Ziegler's opinion that polydactylsm is due to a germ variation : 

 this ?nust be so whenever it is hereditary, for it would not other- 

 wise be transmissible. 



In this way we can also understand why polydactylism, after 

 it has once arisen and has been transmitted through several 

 generations, may finally disappear ; for at every fresh ' reducing 

 division' the number of abnormal ids is increased or decreased, 

 and in the latter case their effect may be entirely obliterated, in 

 consequence of their meeting with perfectly normal ids during 

 the process of amphimixis; and in the next generation they 

 may be for ever eliminated. Like all individual variations, they 

 may be absent from one generation and appear again in the 

 next ; but in case of continual crossing in normal human beings, 

 theoretically they may be expected once more to disappear com- 

 pletely. This agrees with fact, for supernumerary fingers have 

 never been observed in more than five consecutive generations. 



We know that variation consists not only in the addition of 

 parts, but also in their disappearance. The process of degenera- 

 tion of parts must be attributed to the disappearance of the 

 respective determinants from the germ-plasm. In the chapter 

 on reversion, I have already attempted to show that regressive 

 transformations need not occur in all the ids of the germ-plasm 

 at the same time, and that reversion to long-lost ancestral 

 characters may be ascribed to the preservation of a minority 



* Dr. R. Zander has recently declared himself in favour of the view that 

 supernumerary fingers, &c., are formed by the mechanical constriction of 

 the rudiments of the embryonic fingers by amniotic threads. But if this 

 is true, they would not be transmissible, and another explanation must be 

 made for the doubling of the tarsus in beetles. (C/C ' 1st die Polydactylie 

 als theromorphe Varietat oder als Missbildunganzusehen? ' Virch, Arch., 

 Bd. 125, 1891, p. 453.) 



