496 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
find in the whales and sea-cows—two widely separated stocks—a 
horizontal caudal fin, rudimentary hind limbs, fin-shaped fore limbs, 
a more or less naked skin, etc. Let us, however, look more closely 
at one of these structures which appear so similar. The flipper of 
a marine mammal is always much broader than the fore limb of a 
land mammal. The fin must be broad if it is to serve as a good 
steering apparatus. In the whales the broadening of the fin is 
accomplished by the broadening of both lower arm bones, so that the 
bones themselves are increased in breadth antero-posteriorly. The 
space between the radius and ulna is small. (Fig. 27, I.) In the 
sea-cows, on the contrary, the broadening of the fin results from the 
increased space between the radius and ulna. The radius is bent 
forward strongly, while the bones maintain nearly their original 
size. (Fig. 27, II.) In the seals, finally, the same result is reached 
Fie. 27.—The left lower arm-bones of a toothed whale (Fig. I), a 
manatee (Fig. Il), and a seal (Fig. III), seen from without. R., 
radius; U., ulna. 
by the radius being increased in width at its lower end and the ulna 
at its upper end, while the spaces between the bones remain very 
small. (Fig. 27, IIT.) 
Thus the same result, the formation of a broad fin, comes about 
in three different ways. We can state that in briefer form somewhat 
as follows: Like form, with different structure. 
To follow the relations between the form of the body and the 
structure of the body in all particulars is essential if one wishes to 
approach the problem of the affinities of animals one to another. The 
history of the separate stems of the marine mammals must be con- 
structed on foundations entirely different than those upon which the 
similarity of the form of the body is based. We recognize here again, 
as everywhere, the dominating influence of mode of life on the struc- 
ture of organisms, which in different groups produces the phenomena 
of convergent adaptation. 
