38 Darwin, and after Darwin. 



or, according to our theory, in the work of tracing 

 lines of pedigree. But now, the converse of this 

 statement holds equally true. For it often happens 

 that adaptive structures are required to change in 

 different lines of descent in analogous ways, in order 

 to meet analogous needs ; and, when such is the case, 

 the structures concerned have to assume more or 

 less close resemblances to one another, even though 

 they have severally descended from quite different 

 ancestors. The paddles of a whale, for instance, most 

 strikingly resemble the fins of a fish as to their out- 

 ward form and movements ; yet, on the theory of 

 descent, they must be held to have had a widely 

 different parentage. Now, in all such cases where 

 there is thus what is called an analogous (or adaptive) 

 resemblance, as distinguished from what is called an 

 homologous (or anatomical) resemblance in all such 

 cases it is observable that the similarities do not 

 extend further into the structure of the parts than it 

 is necessary that they should extend, in order that the 

 structures should both perform the same functions. 

 The whole anatomy of the paddles of a whale is quite 

 unlike that of the fins of a fish being, in fact, that of 

 the fore-limb of a mammal. The change, therefore, 

 which the fore-limb has here undergone to suit it to 

 the aquatic habits of this mammal, is no greater than 

 was required for that purpose: the change has not 

 extended to any one feature of anatomical significance. 

 This, of course, is what we should expect on the 

 theory of descent with modification of ancestral char- 

 acters ; but on the theory of special creation it is not 

 intelligible why there should always be so marked a 

 distinction between resemblances as analogical or 



