XXX.] CLASSIFICATION^. 683 



so, though, practically, any one who met with a character- 

 istically mammalian jaw would be justified in expecting 

 to find the characteristically mammalian occiput asso- 

 ciated with it ; yet, he would be a bold man indeed, who 

 should strictly assert the belief which is implied 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." 



One of the most distinct and remarkable instances of 

 correlation in the animal world is that which occurs in 

 ruminating animals, and which could not be better stated 

 than in the following extract from the classical work of 

 Cuvier :^ 



" 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 sufticient cause ; but since we are ignorant of 

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

 of observation : it enables us to estal)lish empirical laws 

 which become ahnost as certain as rational laws when 

 they rest on sufficiently repeated observations ; so that 

 nx)w whoso sees merely the print of a cleft toot may con- 

 clude that the animal which left tliis impression rumi- 

 nated, and this conclusion is as certain as any other n\ 

 pliysics or morals. Tliis footprint alone then, yields, to 

 liini wlio observes it, the form of tiie 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 em])irical 

 correhition of circumstances when we classify the planets 

 according to their densities and periods of axial lotation.''^ 

 If we examine a table specifying the usual astronomical 

 elements of the solar system, we find that four planets 



' Ossemens Fossiles, ^.ih edit. vol. i. p. 164. Quoted by Hi'.xley, 

 Lectures, &c., p. 5. 



■^ Chaiiihiis, Descriptive Astronomy, ist edit. p. J3. 



