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



lowest species of that order, it begins to present that condition 

 of the generative organs which characterizes the last section of 

 Mammals (the Marsupiata)^ and which is there accompanied with 

 other characters approaching the oviparous types. These facts 

 and conclusions relating to the Biscacha induce me to place the 

 genus to which it belongs in that part of the circle representing 

 the Rodent order which is nearest to the circle of the Marsupiata. 

 But I cannot place the Wombat {Phascolomys) in the corre- 

 sponding part of the Marsupial circle without observing, that it 

 appears to me its relationship to the Rodent group is of a different 

 nature (or at least differs in degree) ; that it is only in what has 

 been aptly termed ^adaptive characters' that its approach is 

 evinced. These adaptive characters (which I conceive are by no 

 means necessarily connected with affinity) consist in a superficial 

 resemblance, owing to certain similar modifications of organs 

 connected with the habits of the species : thus the Flying Lemur 

 (Galeopithecus), Flying Squirrel (Pteromys), andYljing Phalanger 

 (Petaurus) have a considerable resemblance to each other, arising 

 from each being adapted to a mode of life which is in some re- 

 spects the same in all, but the groups to which the three animals 

 belong are in important zoological characters essentially different; 

 yet it must be observed, that as the Rodents and Marsupiata are 

 more near to each other than either are to the Lemurid(e, there 

 exists a difference of degree as regards the extent of the hiatus 

 which separates the three flying animals referred to : so it is I 

 believe with the Wombat ; it resembles the Rodents in certain 

 adaptive characters, and the approach to the Rodents is only in 

 degree equal to the approach of the order Rodentia to the order 

 Marsupiata. The Lagostomus not only possesses characters which 

 link the Rodentia to the Marsupiata generally, but goes beyond 

 other species of its order in having a modification of its genera- 

 tive system which approaches it still more nearly to the last-men- 

 tioned group. The Wombat even in dentition agrees essentially 

 with the Marsupial type, and not, as was supposed, with the 

 Rodent. 



It is in cases like one or the other of the two which I have 

 endeavoured to illustrate, that I believe the several genera intro- 

 duced in either of the circles of my table evince an approxima- 

 tion to other circles. I do not perceive that the orders imper- 

 ceptibly blend into each other, nor am I at all satisfied that even 

 in minor groups (such as families and genera) this perfect blend- 

 ing takes place. 



The question which arises from such a position is, whether any 

 species is formed essentially on two types of the same rank ? 

 Each animal is framed to perform certain functions, and is most 

 perfectly adapted to those functions ; but beyond this, is not each 



