Page:Popular Science Monthly Volume 68.djvu/342

338 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 metacarpal bones. Paleontology completes the ring of circumstantial evidence

 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.

by showing us that the ancestors of the swine had five instead of four toes and that the forerunners of the ruminants and the Equidæ 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 duplication, 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