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 

 The meaning of ■ , ^i ^ c • i i t r i it/- 



* With that of my neighbour, I find differ- 

 homology. . j o ? 



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 



