Mr. G. R. Watcrhouse on the Classification of Mammalia. 411 



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have more than ^^ri ^^^'^ molars, and the decrease from this 



number to f^-j, or ^ ~ ^ , or q^^, is important, inasmuch as, 

 with but one or two exceptions, it is combined with other cha- 

 racters indicative of the great divisions in that group. 



The Pachydermata vary much in their dentition, in some there 

 being the four kinds of teeth well developed and greatly resem- 

 bling that condition which characterizes the first great carnivorous 

 and frugivorous section, as in the genus Sus, to which the little 

 insectivorous animals forming the genus Centetes bear consider- 

 able resemblance in the general structure of the skull and the 

 greatly developed canines, as well as in external characters. 

 Other Pachyderms again (as the Horses) approach the Ruminants 

 in a very marked degree ; and lastly the Elephants, though linked 

 with the ordinary Pachydermata through the extinct Mastodons, 

 &c., differ remarkably from the normal species in their dentition, 

 in which there is an approach to that of the Rodeilts ; an ap- 

 proach is also perceptible in the sanguiferous system. 



A relationship between the Sloths and the Ruminants is dis- 

 played in the structure of the stomach. 



In all these instances of approach of species of one order to 

 other orders here noticed, there is not a single case which would 

 fairly bear out the notion that these orders imperceptibly blend 

 into each other. There is always a tolerably well-marked line 

 between them (hence I have enclosed the orders in circles). The 

 aberrant species are readily traced back as it were into their own 

 groups, and when they evince an approach to other circles, it is 

 rather to the order than to any particular species of the order. 



In conclusion, I would oJffer the following propositions and ob- 

 servations for consideration : — 



Species of animals belonging to the same genus have an affi- 

 nity to each other ; genera of the same family have a mutual 

 affinity ; relationship of affinity may likewise exist between fami- 

 lies of the same order and orders of the same class, but the de- 

 gree of affinity is different in the different cases ; it is more or less 

 remote. Thus species of the same genus have an affinity of the 

 first or nearest degree ; 



species of different genera of the second degree only ; 

 of different families of the third degree ; 



— of different orders of the fourth degree ; 

 of different classes of the fifth degree. 



A relationship may exist between species of different groups, 

 which differs from either of the cases just mentioned; that which 

 is commonly termed by naturalists a relationship of analogy. 

 This again may vary in degree according to the affinities and 



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