ti2 EVOLUTION AND DISEASE. 
of a bone resembling a supernumerary coracoid. To 
judge from published cases it would seem that super- 
numerary limbs in amphibia are uncommon, but inquiry 
satisfies me that they are, in frogs and toads at least, by 
no means infrequent. 
We will turn from amphibians to birds. In these 
highly specialized and exquisite forms, dichotomy is 
very common. The Dorking fowl has long attracted 
Fic. 60.—A Frog (Rana temporaria), with a supernumerary 
fore-limb. 
attention in that it presents almost constantly a 
double digit on the pes, increasing the number to five. 
This extra toe is due to dichotomy of the digit attached 
to the vestigial first metatarsal; it possesses three 
phalanges, and is furnished with a claw. This deviation 
from the number normal among fowls is further interest- 
ing in that it is transmitted truly to offspring when the 
Dorking is crossed with breeds furnished with four toes. 
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