ii2 E VOL UTION 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 



FIG. 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. 



