4 FOOT-NOTES TO EVOLUTION. 
inside of an animal which tells the real history of its 
ancestry; its outside tells us only where its ancestors 
have been. 
It is perfectly certain that homology represents some 
real law of Nature, something other than the results of 
mere chance. When I compare my arm 
with that of my neighbour, I find differ- 
ences in size and proportions, But these 
are superficial, and there is the underlying correspond- 
ence of each bone and muscle, each nerve fibre, artery, 
and vein, When I compare my arm with the fore leg of 
a dog I find more striking differences, for the dog’s 
station in life is quite unlike my own, and he uses his 
arm for different purposes. When I compare my arm 
with the wing of a bird or the pectoral fin of a fish, the 
results are still similar. Though the differences in each 
succeeding case become more and more striking, and 
the resemblance less easy to trace, yet the same re- 
semblances exist, and a closer study shows that these 
resemblances far outweigh the differences. 
We say, then, that homology is real, and whatever 
power or cause has acted on fishes to provide them with 
pectoral fins has given to birds wings, to the dog fore 
legs, and to me and my neighbour arms. The arms are 
appendages more specialized—that is, more highly fin- 
ished and suited to more purposes than the others—but 
all are formed of the same pieces, arranged in the same 
way. When I compare my arm, however, with the claw 
of a lobster, the limb of a tree, or the arm of a star- 
fish, all resemblances in gross structure disappear, and 
we have only the analogies connected with similar- 
ity of function. The ultimate homology of cell for 
cell, however, remains even here with all that this may 
signify. 
Now the problem before us is this: What is the 
The meaning of 
homology. 
