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



have more than ^-zri ^^^^ molars, and the decrease from this 



number to 3^7, or j ~ - ^ , or ^"E^, 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 Rodents ; 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 07-der than to any particular species of the order. 



In conclusion, I would offer 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 natm-alists a relationship of analogy. 

 This again may vary in degree according to the affinities and 



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