It is said, in short, that a natural-history class is not 

 capable of being defined that the class Rosacese, for 

 instance, or the class of Fishes, is not accurately and 

 absolutely definable, inasmuch as its members will pre- 

 sent exceptions to every possible definition ; and that 

 the members of the class are united together only by 

 the circumstance that they are all more like some 

 imaginary average rose or average fish, than they 

 resemble anything else. 



But here, as before, I think the distinction has arisen 

 entirely from confusing a transitory imperfection with 

 an essential character. So long as our information con- 

 cerning them is imperfect, we class all objects together 

 according to resemblances which we feel, but cannot 

 define; we group them round types, in short. Thus, 

 if you ask an ordinary person what kinds of animals 

 there are, he will probably say, beasts, birds, reptiles, 

 fishes, insects, &c. Ask him to define a beast from a 

 reptile, and he cannot do it ; but he says, things like 

 a cow or a horse are beasts, and things like a frog or a 

 lizard are reptiles. You see lie does class by type, and 

 not by definition. But how does this classification differ 

 from that of the scientific Zoologist ? How does the 

 meaning of the scientific class-name of "Mammalia" 

 differ from the unscientific of "Beasts"? 



Why, exactly because the former depends on a defi- 

 nition, the latter on a type. The class Mammalia is 

 scientifically defined as " all animals which have a ver- 

 tebra ted skeleton and suckle their young." Here is no 

 reference to type, but a definition rigorous enough for a 

 geometrician. And such is the character which every 

 scientific naturalist recognises as that to which his classes 



species than with any others, form the {rcnus, and are ranged about it, 

 deviating from it in various directions and different decrees." WIIE 

 The Philosophy of the Inductive Sciences, vol. L pp. 47(5, 477. 



