42 EVOLUTION 



resemble one another in having a similar use, 

 in discharging the same function, they are 

 said to be analogous. This distinction of the 

 two kinds of likeness, which are confused in 

 popular thought and language, is of far- 

 reaching importance. The discipline of 

 comparative anatomy, largely by help of the 

 Platonic idea of the "archetype" the es- 

 sential or ideal form of each group or species 

 had made the idea of homology clear 

 before it reached its evolutionist interpre- 

 tation; and research increasingly showed 

 that if classification is to be a grouping to- 

 gether of forms that are deeply alike, it must 

 rest on a recognition of homologies, and that 

 a grouping according to analogical resem- 

 blances is bound to be fallacious. 



Aristotle (384-322 B. c.) recognized real 

 kinship when he ranked whales with mam- 

 mals, not with fishes; and bats with mammals, 

 not with birds. And from that early date 

 till now the successful classifiers of animals 

 or of plants have been those who saw clearly 

 through all deceptive suggestions of func- 

 tional resemblance (analogy), and got down 

 to the sure foundation of structural and 

 developmental resemblance (homology). 



To make the distinction between homol- 

 ogies of essential form and mere analogies 

 of use more concrete, let us recall the three 



