CLASSIFICATION AND ADAPTATION 9 



such as colour and patterns, or scales and armour, 

 which may be useful in the life of an animal or 

 flower, but are not mechanisms of moving parts 

 like a bird's wing, or secreting organs like mammary 

 glands. Unless we choose or invent some new 

 term, we must define adaptations apart from all 

 questions of evolution as any structures or characters 

 in an organism which can be shown either by their 

 mere presence, or by their active function, to be 

 either useful or necessary to the animal's existence. 

 We must be on our guard against assuming that the 

 word ' adaptation ' implies any particular theory or 

 conclusion concerning the method and process 

 by which adaptations have arisen in the course of 

 evolution. It is that method and process which we 

 have to investigate. 



On the other hand, when we look primarily at 

 differences of structure we find that not only are 

 there wide and distinct gaps between the larger 

 categories, such as mammals and birds, with few 

 or no intermediate forms, but the actual individuals 

 most closely similar to one another naturally and 

 inevitably fall into distinct groups which we call 

 kinds or species. The conception of a species is 

 difficult to define, and authorities are not agreed 

 about it. Some, like Professor Huxley, state that a 

 species is purely a mental conception, a generalised 

 idea of a type to which actual individuals more or 

 less closely conform. According to Huxley, you 

 cannot lock the species ' horse ' in a stable. Others 

 regard the matter more objectively, and regard the 

 species merely as the total number of individuals 

 which possess a certain degree of resemblance, 

 including, as mentioned above, all the forms which 



