Reviews — American Equidce. 129 



in correlating these with marine deposits. The extremely careful 

 collecting carried out by the American Museum parties has 

 established these zones on a firm stratigraphical basis, and has even 

 allowed of a record of the jDosition of individual specimens within 

 their zone. Thus much of President Osborn's material fulfils in 

 the highest degree the first demand of a palaeontologist. Of the 

 Oligocene genus Mesohippus eighteen species are described, the 

 upper teeth of nine of which, figured in order of age, show a steady 

 advance in structure by stages which are individually scarcely recog- 

 nizable, until the latest form passes into the genus Miohippiis, the 

 line separating the two genera being of jjurely arbitrary position. 

 The upper teeth of seven species of Miohippus, figured in order of 

 age, show an exactly similar steady change, until the latest form 

 M. gemmarosce differs scarcely at all from the most primitive 

 Parahippus, P. pristinus. From this form there is a complete 

 structural passage to such advanced forms as P. hrevidens, which 

 differs very little from the earlier Merychippi, such as AI. isonesus 

 primus. Of this species there is a series of five mutants, in Waagen's 

 sense, not that of de Vries, occurring in order in about 25 feet 

 of sediments, the latest of which, M. i. quintus, differs much more 

 in tooth-structure from M. i. primus than the latter does from the 

 advanced Parahijjpi. More advanced species of the very complex 

 genus Merychijjpus lead insensibly into Protohijjpus, Pliohippus, 

 and Hijjparion, and many stages of the later evolution of these genera 

 can be traced in the material. 



There are thus known about three dozen stages between the 

 most primitive Mesohippus and the most advanced Plioliipptis, 

 and the gaps separating these stages are all of extremely small 

 magnitude, so small that when more material is published they will 

 inevitably disappear entirely, because the structures change steadily 

 with time, and the size of the structural gaj) between two specimens 

 depends on the time interval which separates them. The present 

 work being concerned with type-specimens does not include those 

 forms of intermediate structure which are not of specific or good 

 mutational rank. 



These steady changes in definite directions are shown not only 

 in the teeth but in skull structure and proportions, and especially 

 clearly in the interlocking relations of the carpals and tarsals. 



The fundamental conclusions which, although they are never 

 mentioned in the text, are forced on the reader of this monograph, 

 especially by the comparative j)lates, are : that the evolution of 

 the horses has proceeded steadily with time by the steady 

 accumulation of changes of infinitely small magnitude, without or 

 with only the extremely rare and unimportant occurrence of 

 saltations : that the changes are steadily in the same direction 

 until a character is fully formed and functional : that series of 

 changes of identical character go .on in distinct but parallel phyla, 

 which may in some cases be traced through from genus to genus. 



VOL. LVII. — NO. III. 9 



