in EVOLUTION OF HORSES 151 



directions. Neither do they indicate 

 steps of infinitesimal minuteness. On 

 the contrary, they indicate a steady pro- 

 gress in one determinate line of develop- 

 ment, a progress so rapid that sometimes 

 the new species seem to have been 

 actually living as contemporaries with 

 the older species ; and alongside of the 

 anterior forms which were, as it were, 

 going out of fashion, and are now assumed 

 to have been their own progenitors. 

 The number, too, of the forms through 

 which the line of modifications can be 

 traced during a geological period of 

 apparently no long duration, indicates at 

 that time an activity in the production of 

 new specific characters which is highly 

 suggestive of comparatively rapid changes 

 in the processes and in the products of 

 ordinary generation. Sedimentary beds 

 not exceeding 180 feet in total thickness, 

 and thus indicative of no very long time 



