356 THE PRINCIPLES OF SCIENCE. 



than in the following extract from the classical work of I 

 Cuvier e : 



I doubt if any one would have divined, if untaught I 

 by observation, that all ruminants have the foot cleft, I 

 and that they alone have it. I doubt if any one would I 

 have divined that there are frontal horns only in this 1 

 class : that those among them which have sharp canines j 

 for the most part lack horns. 



However, since these relations are constant, they must I 

 have some sufficient cause ; but since we are ignorant of I 

 it, we must make good the defect of the theory by means j 

 of observation : it enables us to establish empirical laws I 

 which become almost as certain as rational laws when j 

 they rest on sufficiently repeated observations ; so that j 

 now whoso sees merely the print of a cleft foot may con- 1 

 elude that the animal which left this impression rumi- 1 

 nated, and this conclusion is as certain as any other in I 

 physics or morals. This footprint alone, then, yields to j 

 him who observes it, the form of the teeth, the form of I 

 the jaws, the form of the vertebra?, the form of all the j 

 bones of the legs, of the thighs, of the shoulders, and of 

 the pelvis of the animal which has passed by : it is a 

 surer mark than all those of Zadig. 



We meet with a good instance of the purely empirical 

 correlation of circumstances when we classify the planets 

 of the solar system according to their densities or periods 

 of axial rotation f . If we examine a table specifying 

 the usual astronomical numbers of the solar system, we 

 find that four planets resemble each other very closely 

 in the period of axial rotation, and the same four planets 

 are all found to have high densities, thus : 



Ossemens Fossiles, 4th edit, vol. i. p. 164. Quoted by Huxley, 

 Lectures, &c., p. 5. 



f Chambers, Descriptive Astronomy, ist edit. p. 23. 



