Analogies 105 



When we speak of the wings of a bat, the resemblances 

 to the wings of a bird are more numerous. We see especially 

 that in both animals the wings do actually serve for loco- 

 motion through the air, and that in both cases the wings 

 propel the animal by flapping or fluttering against the air. 

 In the same way the wings of a butterfly or of a mosquito do 

 more than look like wings: they act like wings. We find 

 throughout nature that organs which are quite different 

 from each other in their structure may nevertheless act in 

 the same way. The leg of a horse, for example, is a totally 

 different kind of organ from the leg of a lobster, yet both 

 are legs in the sense that they serve their owners in locomo- 

 tion on a solid surface. That is to say, they both behave in 

 the same way. The humming bird sips the nectar from the 

 flower through its beak, and the butterfly sucks nectar 

 through its " proboscis " — two entirely different kinds of 

 structures do the same kind of work; and the proboscis or 

 trunk of the elephant is a still different kind of organ. 



People sometimes contrast the breathing among dif- 

 ferent kinds of organisms by saying that " the gills are the 

 lungs of the fish," or even that " the leaves are the lungs of 

 the tree." These statements are true only in the sense that 

 gills and lungs and leaves are all alike in having some rela- 

 tion to breathing. It would be just as logical to say that our 

 lungs are our gills, or that the lungs are our leaves. A little 

 observation will show that while the many processes that go 

 on in a living thing are repeated in the different orders of 

 plants and animals, they are carried on by strikingly differ- 

 ent organs. The fact that two or more structurally dif- 

 ferent kinds of organs may nevertheless carry on the same 

 kind of function is spoken of as analogy. Two organs of 

 different kinds of plants or in different kinds of animals that 

 have similar functions are said to be analogous. 



In so far as different kinds of plants or different kinds of 

 animals are adapted to similar conditions, we are likely to 

 find analogies among them. The eyes of insects and the 

 eyes of backboned animals are radically different kinds of 



