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POPULAR SCIENCE MONTHLY 



circumstantial proofs of such reductions are too well known to require 

 more than a brief statement. Rudiments supposed to represent the 

 absent first digit are found in the pes of the dog and the manus of the 

 pig (Figs, 2, A, 3, C). The feet of sheep and cattle exhibit pairs of 

 vestigial bones and hoofs, called the rudiments of digits 2 and 5 ; the 

 splint bones of the horse are believed to be the vestiges of the second 

 and fourth digits. These rudiments are often better developed in the 

 embryo than in the adult. Thus of the dog's hallux only the upper 

 part of the metatarsal bone remains. According to Bonnet, all the 

 skeletal parts of this digit are formed in the embryo. The second and 

 fifth digits of the sheep, represented by mere vestiges of the phalanges, 

 are fully developed in the land). The foot of the adult horse shows 

 only the metacarpals and metatarsals of digits 2 and 4 ; but in the 

 embryo the writer has observed two cartilaginous phalanges on the meta- 

 carpal bones. Paleontology completes the ring of circumstantial evi- 



Fig. 1. X-ray photographs of a child's extremities showing duplication of the fifth digit 

 in both hands and both feet. Va, Vb, the digits produced by duplication. 



dence by showing us that the ancestors of the swine had five instead of 

 four toes and that the forerunners of the ruminants and the Equida? 

 had three, four or five functional digits. 



The question now arises as to whether the occurrence of extra digits 

 on extremities normally possessing less than five toes is due to duplica- 

 tion, as in pentadactyl animals, or are the extra digits developed from 

 the rudimentary structures we have described? If it can be shown 

 that the supernumerary toes are due to reversion, we have no longer 



