VARIATION 429 



insects, are due to the doubling of a group of determinants, 

 and perhaps the much-discussed and debated problem con- 

 cerning supernumerary fingers and toes in human beings may 

 be explained in a similar way. There is nothing impossible in 

 the assumption that the latter phenomenon is due to reversion 

 ' to an extremely remote, lowly-organised, and many-fingered 

 ancestor/ for we know of other cases of reversion to very distant 

 ancestral forms. But in none of the reliable instances does rever- 

 sion to ancestral characters extend through such an enormous 

 lapse of time or immense number of generations as must be 

 assumed in this case. The striping on mules points back to an 

 early equine ancestor, and we are led to the conclusion that at 

 the present day the germ-plasm of horses and asses still contains 

 solitary '■ zebra' determinants. Reversion may occur to yet more 

 remote ancestors of the modern horse, — even to those possessing 

 three toes ; but cases of reversion to still earlier ancestors can 

 hardly be proved with any degree of certainty ; nor is it probable, 

 from a theoretical point of view, that any groups of determinants 

 of such extremely remote primitive mammals should have been 

 preserved in the germ-plasm of human beings. Moreover, it is 

 not at all certain that the primitive mammals possessed more 

 than five fingers and toes ; and it would be necessary to go back 

 to much more remote ancestors before obtaining any support for 

 the explanation of human polydactylism given by Darwin, and 

 formerly accepted by Bardeleben, Wiedersheim, and others. 



Not only is there no firm foundation for this latter assumption, 

 but there appear to me to be very weighty reasons against it. 

 We must not, in the first place, overlook the fact that these primi- 

 tive ancestors did not possess ' human ' fingers ; supernumerary 

 fingers are, nevertheless, real fingers, and though they are not 

 always perfect, they are furnished with the form of nail typical 

 of the human finger, and not with claws. In my opinion, we are 

 not justified in assuming that such a supernumerary finger is 

 represented in the germ-plasm by a group of determinants de- 

 rived from the primitive ancestor, and that this group has in the 

 meantime become transformed into the type of the human finger. 



Apart from polydactylism, cases of the doubling of the limbs 

 are known, which, from their nature, cannot be looked upon as 

 atavistic : insects, for example, have never possessed a double 

 tarsus. There must, therefore, be another way in which this 

 doubling might originate. 



