HI LECTURES ON EVOLUTION 119 



The teeth of a horse are not less peculiar than 

 its limbs. The living engine, like all others, must 

 be well stoked if it is to do its work ; and the 

 horse, if it is to make good its wear and tear, and 

 to exert the enormous amount of force required 

 for its propulsion, must be well and rapidly fed. 

 To this end, good cutting instruments and power- 

 ful and lasting crushers are needful. Accordingly, 

 the twelve cutting teeth of a horse are close-set 

 and concentrated in the fore-part of its mouth, 

 like so many adzes or chisels. The grinders or 

 molars are large, and have an extremely compli- 

 cated structure, being composed of a number of 

 different substances of unequal hardness. The 

 consequence of this is that they wear away at 

 different rates ; and, hence, the surface of each 

 grinder is always as uneven as that of a good 

 millstone. 



I have said that the structure of the grinding 

 teeth is very complicated, the harder and the 

 softer parts being, as it were, interlaced with one 

 another. The result of this is that, as the tooth 

 wears, the crown presents a peculiar pattern, the 

 nature of which is not very easily deciphered at 

 first ; but which it is important we should under- 

 stand clearly. Each grinding tooth of the upper 

 jaw has an outer wall so shaped that, on the worn 

 crown, it exhibits the form of two crescents, one 

 in front and one behind, with their concave sides 

 turned outwards. From the inner side of the 



