INTRODUCTION. 



THE teeth are the hardest of the tissues, and, unlike other tissues, are 

 not improved, but on the contrary constantly worn away and finally 

 destroyed, by prolonged use. Yet they are also the most progressive 

 of the tissues. For example, in the evolution of the horse, no other 

 system of organs undergoes so profound a change as that of the teeth. 



The fitness which they present for every possible mode of capturing 

 and eating fills volumes of the older works on descriptive anatomy and 

 teleology, and is even now constantly disclosing new and fascinating 

 subjects for the study of adaptations. 



Because of their hardness the teeth are the most generally and 

 perfectly preserved of all fossilized organs ; hence they are the especial 

 guides and friends of the palaeontologist in his peculiar field of work 

 from imperfect evidence. Thus it happens also that the palaeontologist 

 has been obliged to study the teetli in more detail even than has been 

 done by the comparative anatomist or zoologist. 



All this use of the teeth for teleological, descriptive, and taxonornic 

 purposes is, however, entirely aside from the main purpose of the present 

 volume, which is, to set forth the mode of evolution cruelly of the 

 complex crowns of the molar teeth of mammals, how the main types 

 originated, and how they can be compared with each other and with 

 those of reptiles. The evolution is not, however, traced to its final 

 modifications in the elaborate hypsodont and tubercular types, but only 

 so far as the formation of the fundamental patterns. 



That comparison can be made on a grand scale most attractive to 

 the student, both of homologies and analogies, is a recent discovery. 

 It dates back only to the year 1883, when what may be called the 

 law of trituberculy was discovered by Cope. 1 No harmony existed in 

 our ideas and descriptions of the grinding teeth of mammals previous to 

 that time, although Huxley in his discussion of the teeth of the Insec- 

 tivora had anticipated the discovery of such a harmony. Many of 

 the details and of the broader outlines of the law were either touched 



1 American Xatwalist, April 1883, pp. 407-408. 

 A 



