Heredity and Natural Selection. 309 



It is sometimes difficult to determine whether a resem- 

 blance is an homology or not, and in simple cases we 

 decide by asking whether it can be due to similarity of 

 use. We know that a bird's wing is more closely related 

 to a man's arm than it is to an insect's wing, because the 

 resemblance between the two wings is no more than we 

 should expect in organs adapted to the same purpose; 

 but there is nothing in the use of the arm and wing to 

 explain what they have in common. 



In cases too complicated to be settled in this simple 

 way we appeal to embryology, and ask whether the re- 

 semblance becomes more marked or less marked when 

 we study it in its younger stages. The arm and the wing 

 are more alike in the embryo than they are in the adults, 

 and the features which they share in common make their 

 appearance earlier than their distinctive characteristics. 



An homology then is a resemblance which is not due 

 to similarity of use, and which is more conspicuous in 

 the embryo than in the adult. 



This is the doctrine of homology considered from its 

 structural side; historically considered, an homology is 

 a resemblance due to community of descent, as distin- 

 guished from one due to recent modification. The 

 modern morphologist believes that the resemblances 

 between a bird's wing and a man's arm are due to in- 

 heritance from a remote ancestor in which the limb 

 had all the characteristics which are common to the 

 wing and arm; that during the evolution of birds and 

 mammals along two divergent lines from this ancestral 

 form, the distinctive features which fit the wing for 

 flight and the hand for grasping have been gradually 

 acquired. 



The doctrine that homology is an indication of ances- 

 tral relationship, and that the past history of organisms 



