MODES OF MAMMALIAN EVOLUTION 661 



is that of continuity, but a closer analysis reveals numerous 

 small breaks, and suggests, so far as the record may be trusted, 

 that the advance was made by separate steps, though very 

 short ones. Indeed, it has been objected that so completely 

 recorded a phylum as that of the horses must be illusory, be- 

 cause there is not perfect continuity between the successive 

 genera, it being taken for granted that such continuity is the 

 normal mode of development. 



Dr. Schlosser, on the other hand, is a disbeliever in perfect 

 continuity. ''I am of the opinion that we must reckon with 

 development per solium more frequently than is usually done. 

 We have been decidedly spoiled by the phylogenetic series of 

 quiet successive development, such as we meet with in the 

 OHgocene and Miocene of North America in the titanotheres, 

 oreodonts, camels, etc., and in the upper Eocene of Europe in 

 Palceotherium, Paloplotherium, etc., as well as from the OHgocene 

 into the Pleistocene, e.g., in the rhinoceroses, cervids, suillines, 

 amphicyonids. Even here we often make for ourselves arti- 

 ficial difficulties by balancing, with an exaggerated scrupu- 

 lousness, the individual forms one against another, to see 

 whether they really are exactly fitted to fill up any gaps. It is 

 not the lack of suitable intermediate forms which so often 

 renders difficult the establishment of genetic series, but, quite 

 on the contrary, the abundance of the forms at our disposal, 

 among which we must make a choice. If, however, the develop- 

 ment of phyla did not take place in the same region and under 

 constant climatic and topographical conditions, we must 

 necessarily find apparent gaps, for adaptation to a new environ- 

 ment occasions rapid changes of organization, so that the 

 immediate descendant will often deviate considerably from its 

 ancestor. But that must not mislead us into denying the 

 connection between such forms." ^ 



Better adapted to a solution of this problem than mammals 



1 M. Schlosser, Beitrage zur Kenntniss der Oligozanen Landsaugethiere aus 

 dem Fayum, Vienna, 1911, p. 165. 



