8 INTRODUCTION. 



found to go along with the possession of four legs in the regu- 

 lar quadrupeds. This will bring in the snakes and some of 

 the fishes. So that, paradoxical as it may seem, it is in one 

 sense scientifically correct to speak of a snake as a quadruped, 

 though in reality it has no legs at all. In other words, there 

 is no reason why a snake should not some day be found with 

 four legs, and in point of fact some snakes show rudiments of 

 these appendages. Making these allowances, and some more 

 of a similar nature, we may ultimately succeed in converting 

 our division of Quadrupeds into a strictly scientific group, 

 comprising the Mammals, the Birds, the Reptiles, the Amphi- 

 bia, and the Fishes. In fact, our group of Quadrupeds now 

 agrees exactly with the great and natural division of the Ver- 

 tebrata or vertebrate animals. It is true that all vertebrate 

 animals have not got four limbs, or not obviously so, but they 

 never have more than four under any circumstances ; and a 

 closer examination soon shows us that they agree with one 

 another in many other characters which are of much greater 

 importance than the characters of the limbs alone. 



We have arrived, then, at the grand principle of all good 

 classification namely, that we should group together those 

 objects only which are united by essential and fundamental 

 points of similarity, and that in so doing we should ignore all 

 mere superficial resemblances. The question now arises, 

 What are these essential and fundamental points in the case 

 of animals ? 



If for the moment we look at animals simply as so many 

 machines, we shall not find much difficulty in answering this 

 question. Let us suppose ourselves placed in a gigantic 

 workshop full of an immense number of complicated and curi- 

 ously-constructed machines of different sorts, and asked to put 

 them in order to put those of one kind in one place, and 

 those of another kind in a different place. How should we 

 proceed to act ? Supposing, in the first place, that all the 

 machines were at a stand-still, all that could be done would 

 be to examine carefully the external form and internal struct- 

 ure of each, and to do our best to pick out some peculiarity 

 which would distinguish some from all the others. In this 

 way, if our mechanical knowledge were sufficiently extensive, 

 we should no doubt ultimately succeed in classing all our ma- 

 chines into something like a rough natural arrangement. We 

 should, for instance, have those made on the principle of the 

 lever in one place, those on the principle of the inclined plane 

 in another, and those on the principle of the pulley in a third. 



