682 THE PRINCIPLES OF SCIENCE. [CHAP. 



of bone, articulated with the squamosal element of the 

 skull, and also with the possession of mammae and non- 

 nucleated red blood-corpuscles. Professor Huxley remarks l 

 that this statement of the character of the class mammalia 

 is something more than an arbitrary definition ; it is a 

 statement of a law of correlation or co-existence of animal 

 structures, from which most important conclusions are 

 deducible. It involves a generalisation to the effect that 

 in nature the structures mentioned are always found 

 associated together. This amounts to saying that the 

 formation of the class mammalia involves an act of induc- 

 tive discovery, and results in the establishment of certain 

 empirical laws of nature. Professor Huxley has excellently 

 expressed the mode in which discoveries of this kind enable 

 naturalists to make deductions or predictions with con- 

 siderable confidence, but he has also pointed out that such 

 inferences are likely from time to time to prove mistaken. 

 I will quote his own words : 



" 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 con- 

 fidence, that the animal of which it formed a part 

 suckled its young, and had non-nucleated red blood-cor- 

 puscles ; and to predict that should the back part of that 

 skull be discovered, it will 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 im- 

 press 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 observa- 

 tional 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 would have been obvious enough. And 



1 Lectures on the Elements of Comparative Anatomy, and on the 

 Classification of Animals, 1864, p. 3. 



