INTRODUCTION. 7 



arrangements, animals were grouped in accordance with some 

 particular character, which might or might not be a really 

 essential one ; and the result was that these classifications 

 were "artificial," and not "natural," as they are when all the 

 characters are taken into consideration. To take a famihar 

 example of this : when we speak of " quadrupeds," we really 

 do so in consequence of our having, consciously or uncon- 

 sciously, formed something like a rough classification of the 

 animal kingdom. We have a dim idea that all animals with 

 four legs belong together somehow, and form a single group. 

 Our classification, however, is founded upon a single character 

 only — ^the possession, namely, of four legs ; and it is, there- 

 fore, a purely artificial arrangement. It will, however, be 

 practically good or bad, just as this single character expresses 

 a genuine and fundamental distinction, or is of a merely trivial 

 and superficial nature. The instance here chosen will serve to 

 illustrate either case. If we insist upon the fact that all the 

 four legs must be externally visible, unmistakable legs, never 

 fewer in number than four, then our classification is a very bad 

 one, in fact entirely " artificial." In this case oiu" group of 

 " quadrupeds " will comprise only the ordinary four-legged 

 mammals, such as oxen, sheep, horses, and such-like — together 

 with the very dissimilar groups of the four-legged reptiles and 

 amphibians, such as tortoises, lizards, crocodiles, frogs, and 

 newts. Now, these different animals have certainly much in 

 common, but we are not justified in placing them together 

 simply upon the ground that they have four conspicuous legs, 

 unless we are willing to put in a vast number of other animals 

 as welL We must, in fact, put in a great number of animals 

 which are not quadrupeds in the sense that they have four legs, 

 but which agree with those that have four legs in the other 

 fundamental and essential points of their structure. In this 

 way we may arrive at a very genuine and natural classification 

 by making some concessions. We must allow, for instance, 

 that two of the legs or limbs, ceasing to be fit for walking, 

 may be converted into organs of flight, or wings. This will 

 let in the birds. We must allow, again, that all the limbs 

 may be converted into fins. This admits most of the fishes. 

 We must further grant that two of the legs may be altogether 

 absent while the remaining two are converted into swimming- 

 paddles. This will bring in the whales and dolphins. Lastly 

 — and this is the greatest admission of all — we must allow 

 the total absence of all the limbs, provided the animal only 

 show those other essential characters which are invariably 



