OF THE .CLASS MAMMALIA. 257 



their resemblance to each other, they will at five years resem- 

 ble horses of different breeds : the one will grow up a coach- 

 horse, light and elegant ; the other an enormous animal, 

 hardly able to trot, but equal to the draught of the heaviest 

 loads. Where nourishment abounds, there the size and 

 weight of the horse increase ; opposite circumstances are 

 attended with other results. Fed on strong humid pastures, 

 the horse becomes heavy, and his form and hair coarse ; on 

 light soils, and especially if grain be added to his ordinary 

 food, his form becomes light and elegant : finally, the careful 

 grooming bestowed on the English horse contributes to 

 improve his form, and especially the fineness of his limbs. 

 Thus it happens, as in the dog, that varieties may be so 

 multiplied as to make it appear that they must have sprung 

 from more than one species : nevertheless, all these modifica- 

 tions have their limits, beyond which nature will not or 

 cannot go : they never destroy the distinctive characters of 

 zoological species.* 



409. Classification of the Mammalia. There exist, 

 as we have seen, considerable differences amongst the mam- 

 malia, and these modifications of structure serve as the basis 

 for the division of the class into groups of an inferior rank, 

 called orders. Most of these groups are so distinct as to 

 admit of no doubt in respect of their limits : they constitute, 

 in fact, natural divisions ; but in others the line of demarca- 

 tion is by no means so distinct. Thus a mammal may have 

 points of resemblance so close to two groups as to render it 

 almost indifferent to which it be referred. To some natu- 

 ralists, differences appear important which are disregarded by 

 others, and hence a want of agreement on the subject of 

 classification has always prevailed. 



The method followed here is nearly that proposed by 

 Cuvier. It rests mainly on the differences mammals show 

 in respect of their extremities and teeth, differences which 

 always imply a crowd of others in structure, habits, and 

 even intelligence. 



410. Keeping in view the ensemble of these characters, 

 the class mammalia may be divided into two groups, the 

 monodelphic and didelphic. 



The monodelphic or monodelphian are the more numerous, 



* Experiments made in this country have led to entirely different results. 

 The various breeds of horses, cattle, and sheep remain unaltered, on what- 

 ever pastures they may be fed. R. K. 



S 



