4 ON CLASSIFICATION. 



tion of the class Mammalia is a statement of a law of correlation, 

 or coexistence, of animal structures, from which the most im- 

 portant conclusions are deducible. 



For example : if a fragmentary fossil be discovered, consisting 

 of no more than a ramus of a mandible and that part of the skull 

 with which it articulated, a knowledge of this law may enable 

 the palaeontologist to affirm, with great confidence, that the 

 animal of which it formed a part suckled its young and had 

 non-nucleated red blood-corpuscles ; and to predict that should 

 the back part of that skull be discovered, it mil exhibit two 

 occipital condyles and a well-ossified basi-occipital bone. 



Deductions of this kind, such as that made by Cuvier in the 

 famous case of the fossil opossum of Montmartre, have often 

 been verified, and are well calculated to impress the vulgar 

 imagination ; so that they have taken rank as the triumphs of 

 the anatomist. But it should carefully be borne in mind, that, 

 like all merely empirical laws, which rest upon a comparatively 

 narrow observational basis, the reasoning from them may at any 

 time break down. If Cuvier, for example, had had to do with a 

 fossil Thylacinus instead of a fossil Opossum, he would not have 

 found the marsupial bones, though the inflected angle of the jaw 7 

 would have been obvious enough. And so, though, practically, 

 any one who met with a characteristically mammalian jaw 

 would be justified in expecting to find the characteristically 

 mammalian occiput associated with it ; yet, he would be a bold 

 man indeed, who should strictly assert the belief which is im- 

 plied in this expectation, viz., that at no period of the world's 

 history did animals exist which combined a mammalian occiput 

 with a reptilian jaw, or vice versa. 



Not that it is to be supposed that the correlations of struc- 

 ture expressed by these empirical laws are in any sense acci- 

 dental, or other than links in the general chain of causes and 

 effects. Doubtless there is some very good reason why the 

 characteristic occiput of a Mammal should be found in association 

 with mammae and non-nucleated blood-corpuscles ; but it is one 

 tiling to admit the causal connexion of these phenomena with 

 one another, or with some third ; and another thing to affirm 

 that we have any knowledge of that causal connexion, or that 



