DARWIN. 187 



ever power or influence or cause has acted on 

 fishes to provide them with pectoral fins has given 

 to birds wings, to the dog forelegs, and to me and 

 my neighbor arms. The arms are appendages 

 more highly finished and suited to more purposes ; 

 but all are formed of the same pieces, arranged in 

 the same way, and all bear the stamp of the same 

 maker. But when I compare my arm with the 

 claw of a lobster, the limb of a tree, the arm of a 

 starfish, or an arm of a sea, all resemblances in 

 structure disappear, and we have only chance 

 analogies. 



This then is certain : In Nature homology exists, 

 and among us back-boned animals all structures, 

 all functions, and at least some of the mental 

 operations show distinct homology. The essence 

 of the development theory is this : Homology is 

 the stamp of heredity. Homology means blood 

 relationship. No other meaning has ever been 

 shown, nor is there the slightest evidence that any 

 other interpretation is possible. Blood relationship 

 implies a common action of heredity, and heredity 

 is the only known source of the likenesses we call 

 homology. '* Owing to the imperfection of lan- 

 guage," said Dr. Erasmus Darwin, a century ago, 

 ** the ofi'spring is termed a new animal ; but it is in 

 truth a branch or elongation of the parent, since a 

 part of the embryon-animal is or was a part of the 

 parent, and therefore may retain some of the habits 

 of the parent system." 



I resemble my neighbor so closely that people 

 say that we look like brothers. My little boy shows 

 similar exactness of homology to me, and people 



