1 66 NATURAL HISTORY. 



and spiny ant-eater, in which the angle of the jaw is never- 

 theless not inflected. He did not know that he might 

 have found the inflected angle of the lower jaw, and that 

 he might have been quite right in his conclusion that the 

 animal was a Marsupial ; and yet, on laying bare the 

 pelvis, he might have found no ' marsupial bones.' It 

 is known, namely, that in certain living Marsupials (the 

 Thyladnus of Tasmania) the 'marsupial bones' do not 

 become converted into bone, but remain permanently 

 in the condition of cartilage. These structures would 

 therefore be absent in any fossil specimen of such a 

 Marsupial, since cartilages are not preserved in the 

 fossil state. Hence, it is possible, though not probable, 

 that we might some day meet with the skeleton of some 

 extinct Marsupial, in which we should find the angle 

 of the lower jaw to be inflected, but which would never- 

 theless show no traces of * marsupial bones.' 



In the third place, in any two correlated organs it is not 

 usual that each is correlated with the other, but that one of 

 the two is correlated with the other. That is to say, of 

 any two correlated organs, A and B, it may be true that 

 A is never found without B, but it does not follow that 

 B may not occur without A. Thus, the presence of a 

 stomach adapted for ' rumination ' is invariably associated 

 with an imperfect development of the incisors of the upper 

 jaw, the central upper incisors being always wanting ; but 

 it is not the case that an incomplete condition of the 

 upper incisors, or the absence of the central ones, is 

 necessarily correlated with the habit of chewing the cud. 

 The proper way of putting the case is to assert that 

 certain structures (A) are never found apart from other 



