222 MAMMALIAN DESCENT. [Lect. IX. 



of the three kinds of .stony substance of which they are composeih 

 The cement Avears fastest, the ivory next; the enamel is the hardest, 

 and wears very slowly ; thus elegant ridges of this tooth sub- 

 stance are always seen on the worn surface of the tooth. There the 

 " final purpose " is not far to seek ; the question is how this modifica- 

 tion came about ; — a modification correlated with numberless other 

 differences of structure that run through the whole animal, and are 

 in harmony with one another. Those with the permanently-growing 

 compound grinders (the genus Arvicola) are almost pure vegetable 

 feeders, like the Beaver ; those with simpler grinders, the members 

 of the genus Mus, are omnivorous with a vengeance. The IS'orway 

 Rat is called Miis derumanus, or tithe-farmer ; he does not jiai/ tithes 

 of all he possesses, but he taA'cs tithes of all you possess, and he is 

 as unscrupulous a rogue as any of those men with itching palms 

 whose title he has had conferred upon him. Xow, if a Darwinian 

 were asked whether the difference between the teeth of these two 

 types of Rodents was a case worthy the interposition of the " Deus 

 ex machina," he would be ready to laugh at the simple question. 

 He cannot certainly bring out of their graves the whole kin of the 

 Arvicolan forefatliers to prove that very long ago they had all a 

 simpler kind of grinders, like the grinders of the existing genus Miu^; 

 but if you will accompany him to any Natural History Museum, he 

 will show you every intermediate condition between the two types 

 of grinders, and make it plain to you as to the easiness of the tran- 

 sition. Gentle modifications of this kind ; very small variations 

 accumulating during long secular periods, in every case in harmony 

 with the hal)its of the creature, slowly, or even suddenly, changing 

 with changing conditions of temperature, food, fresh needs for safety, 

 and the like ; — all these things have to be taken into account. 



It is reassuring to find that Goethe held very similar views to 

 those put forward in these lectures. 



Eckermann, that German Boswell, has preserved for us the follow- 

 ing remarks on the doctrine of " purpose," which will bear quoting 

 here : — 



" Goethe has been speaking of the book of a young natural 

 philosopher, which he could not help praising, on account of the 

 clearness of his descriptions, while he pardoned him for his teleo- 

 logical tendency. 



