98 EVOLUTION 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, 
