266 ON THE CLASSES OF THE 



difficulty, so far from being employed to separate the two 

 groups as we might have expected it would be, has, on 

 the contrary, been very properly disregarded by natural- 

 ists. We have only to regret that the union of Mam- 

 malia and Aves should, ever since the days of Linnaeus, 

 have been accompanied by a glaring violation of natural 

 order in the place of the Cetacea, which have been sup- 

 posed to intervene, though certainly few of the Fertebrata 

 are wider apart from the feathered creation. Those birds 

 which appear in their internal as well as external structure 

 to approach the nearest to the Mammalia belong undoubt- 

 edly to the Ostrich family; but then the imagination must 

 be sadly taxed before we can point out any particular qua- 

 druped as meeting them. — Among the Mammalia we find 

 that the Monotrbmes of Geoffroy have an affinity to Birds; 

 but although in these singular animals the rectum, the 

 urethra, and the spermatic canals, have all only one ex- 

 ternal opening ; though there is every reason to believe 

 them to be in some degree oviparous; though they pos- 

 sess the furcate bone of Birds, with many singularities of 

 formation that separate them from the other Mammalia, 

 yet no bird is as yet known to meet them. The Ornitho- 

 rhynchus paradoxus possesses the beak of a duck, it is 

 true ; but there are grounds for thinking that it would be 

 difficult to prove much further particular affinity to exist 

 between them. Besides, the Ornithorhynckus after all is in 

 some respects so true a quadruped, that it becomes almost 

 ridiculous to make it the immediate means of transition; we 

 must therefore leave the problem to be decided by time. Se- 

 veral quadrupeds apparently possess other affinities to Birds, 

 as the Jerboa for instance in its legs, the Bat in its ster- 

 num : but it is more than probable that these similarities 



