8 CLASSIFICATION AND ADAPTATION 



to distinguish between two kinds of becoming 

 namely, the development of the organ in the in- 

 dividual and its evolution in the course of descent. 

 The word * adaptation ' is itself the cause of much 

 fallacious reasoning and confusion of ideas, inas-^ 

 much as it suggests a process rather than a condition, 

 and by biological writers is often used at one time 

 to mean the former and at others the latter. We 

 may take the mammary glands of mammals or 

 organs adapted for the secretion of milk, whose 

 only function is obviously the nourishment of the 

 offspring. Here the function is certain whatever 

 view we take of the origin of the organs, whether we 

 believe they were created or evolved. But if we 

 consider the flipper or paddle of a whale, we see 

 that it is homologous with the fore-leg of a terrestrial 

 mammal, and we are in the habit of saying that in 

 the whale the fore-limb is modified into a paddle and 

 has become adapted for aquatic locomotion. This, 

 of course, assumes that it has become so adapted 

 in the course of descent. But the pectoral fin of a 

 fish is equally ' adapted ' for aquatic locomotion, 

 but it is certainly not the fore-leg of a terrestrial 

 mammal adapted for that purpose. The original 

 meaning of adaptation in animals and plants, of 

 organic adaptation to use another term, is the 

 relation of a mechanism to its action or of a tool 

 to its work. A hammer is an adaptation for knock- 

 ing in nails, and the woodpecker uses its head and 

 beak in a similar way for making a hole in the bark 

 of trees. The wings and the whole structure of a 

 bird's body form a mechanism for producing one of 

 the most difficult of mechanical results, namely, 

 flight. Then, again, there are stationary conditions, 



