HUXLEY, ON CLASSIFICATION, 67 
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, for 
reasoning from them may at any time break down. If Cuvier, the 
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 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 character- 
istically 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 versd. 
Not that it is to be supposed that the correlations of structure ex- 
pressed by these empirical laws are in any sense accidental, 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 mamme and non- 
nucleated blood-corpuscles ; but it is one thing to admit the causal 
connection of these phenomena with one another, or with some third ; 
and another thing to affirm that we have any knowledge of that 
causal connection, or that physiological science, in its present state, 
furnishes us with any means of reasoning from the one to the 
other. 
Cuvier, the more servile of whose imitators are fond of citing his 
mistaken doctrines as to the nature of the methods of paleontology 
against the conclusions of logic and of common sense, has put this so 
strongly that I cannot refrain from quoting his words.* 
“ But I doubt if any one would have divined, if untaught by ob- 
servation, that all ruminants have the foot cleft, and that they alone 
have it. I doubt if any one would have divined that there are frontal 
horns only in this class: that those among them which have shar 
canines for the most part lack horns. 
“« However, since these relations are constant, they must have some 
sufficient cause: but since we are ignorant of it, we must make good 
the defect of the theory by means of observation: it enables us to 
establish empirical laws, which become almost as certain as rational 
* “Ossemens fossiles,’ ed. 4™*, tome 1", p. 164, 
