100 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



other types which have preserved some characters 

 of this last group. When, in a deposit, we no longer 

 meet, in Europe, any but placental Mammals, 

 we are at a more recent period than the Oligocene. 



The study of the teeth offers us, among the 

 Mammals, some interesting characters of evolution. 

 Thus the pre-molars of several families of Pachy- 

 derms were at first triangular before becoming 

 quadrangular by the addition of a fourth point 

 to the three preceding. This modification is noticed 

 in the Tapir, Rhinoceros, Palaeotherium, the Equidae, 

 etc. The triangular or quadrangular stage of the 

 upper pre-molars will thus indicate to us an early 

 or, on the contrary, a more recent age. 



These examples will no doubt suffice to make 

 thoroughly plain the method set forth by Gaudry 

 and applied by him in a more or less happy manner 

 to the study of a certain number of groups of fossil 

 animals. This method will perhaps appear seduc- 

 tive at first sight ; I will even add that it is suscep- 

 tible of rendering service to palaeontologists, by 

 translating, in a few brief and clear formulas, 

 evolutionary tendencies suitable to this or that well 

 defined natural group. But it requires to be 

 handled with extreme prudence under penalty of 

 inducing error. Evolution does not present itself, 

 in fact, with the same orientation in all the groups ; 

 it is, following the idea so happily expressed by 

 Cope, sometimes progressive and sometimes re- 

 gressive, without anything to enable us to foresee 

 this change of direction. To keep to the Mammals 

 only, examples abound with contradictory tendencies. 

 The back molars become more and more complicated 



