98 E VOL UTION AND DISEASE. 



heredity, that in the embryo it to some extent maintains 

 its pristine eminence. This is strikingly shown in birds. 

 In the chick at the fifth day of incubation the fibula 

 equals in length, and nearly in thickness, the tibia. 

 Subsequently it dwindles, and in the adult bird it is 

 represented as a slender style of bone appended to the 

 proximal extremity of the tibia. In man the tibia, as 

 compared with the fibula by weight, is as three to one : 

 at the third month of embryonic life the fibula has a 

 transverse section nearly equal to that of the tibia. 

 Even in adult life if the tibia be broken and fail to 

 unite, extra work is thrown upon the fibula, and in course 

 of time this bone will enlarge, and its shaft, as I 

 have been able to demonstrate, may exceed in thickness 

 that of the tibia. Darwin refers to some experiments 

 of Sedillot in which small portions of the shaft of 

 the tibia were removed in young dogs : the result was 

 that the fibula, which in dogs is almost as slender 

 as in birds, became greatly increased in size conse- 

 quent upon the extra work required of it. As addi- 

 tional evidence in support of the view that the small 

 size of the fibula in comparison with the tibia is due, 

 indirectly, to the change of function of the leg from 

 a paddle to an organ for land locomotion, it may be 

 mentioned that in such aquatic mammals as seals the 

 fibula is not so small in proportion to the tibia as is the 

 case with terrestrial mammals. It is on these grounds 

 that we may reasonably believe that the small size of 

 the fibula, in comparison with the tibia, may be included 

 as one of the changes resulting indirectly from the 

 gradual change of an aquatic into a terrestrial animal. 



