356 THE PRINCIPLES OF SCIENCE. 



than in the following extract from the classical work of 

 Cuvier e : 



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

 by observation, 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 sharp 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 laws when 

 they rest on sufficiently repeated observations ; so that 

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

 clude that the animal which left this impression rumi- 

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

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

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

 the jaws, the form of the vertebrae, the form of all the 

 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. 1 64. Quoted by Huxley, 

 1 Lectures,' &c., p. 5. 



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



