INTRODUCTION 45 



Similarly the slow stages in the attainment of perfection in the grind- 

 ing teeth of the Eocene horses are of great value; as time-keepers; for 

 example, in the molars of Eohippus and Orohippus we observe that in the 

 lower levels a certain cusp is adumbrated in shadowy form; on a slightly 

 higher level it is distinctly visible; on a still higher level it is fully grown. 

 We do not observe any sudden breaks, but a series of minute gradations, 

 always in the direction of adaptation, because it appears that these changes 

 in the teeth, which Osborn has called " rectigradations, " may be of the 

 same kind as those to which Waagen apphed the term "mutations" in 

 observing shells of successive geological levels. Whenever a new character 

 is thus gradually l:)rought to perfection, the animal is assigned a new spe- 

 cific name; Eohippus validus becomes Eohippus venticolus, or Orohippus 

 hallardi passes into Orohippus progressus. When a number of these new 

 characters thus gradually assemble in different parts of the tooth series, or 

 in the feet, we assign a new generic name: Eohippus becomes Orohippus, 

 or Orohippus becomes Epihippus. The specific and generic names which 

 were applied both in Europe and America to the Eocene horses by Owen, 

 Cope, and Marsh were in every case defined by the presence of such slowly 

 evolving new characters or groups of characters. 



Now the time-keeping value of mammals lies in the fact that in Great 

 Britain, in France, in Switzerland, in the Rocky Mountains, in short, 

 wherever these inconspicuous but important 'rectigradations' are appear- 

 ing, they arise at approximately the same rate and approximately in the 

 same order even among animals which are widely separated geographically. 



Close geologic synchronism, moreover, requires a comparison of the 

 entire fauna and entire flora. The survival of a few primitive or arrested 

 types may mislead, as in Australia, for example. Huxley ' was somewhat 

 doubtful of the time-keeping value of fossils; at least he thought the ap- 

 plication might be overdone. He went so far as to say, "It is possible that 

 similar, or even identical, faunae and florjE in two different localities may 

 be of extremely different ages, if the term ' age ' is used in its proper chron- 

 ological sense." Such a possibility as Huxley imagined has never been 

 more than partly realized. Among the mammals as well as among the 

 plants there is a constant progression which is, on the whole, a guide or 

 index to synchroneity. This does not preclude such ]:iroa(l statements as 

 the following: that the general aspect of modern Africa resembles that of 

 Pliocene Europe. 



Various Evidences of Synchronism and Homotaxis 



When we attempt to compare what is going on in the Old and New 

 Worlds during the enormously long time which is called the Age of Mam- 



' Huxley, The Anniversary Address of the President. Quart. Jour. Geol. Soc. London, 

 Vol. XXVI, 1870, pp. 29-64; Scientific Memoirs, Vol. Ill, p. 526. 



