XV111 INTRODUCTION. 



latter extremely closely, are met with in the Triassic, and there 

 are undoubted small mammals in the Jurassic; but these are 

 insignificant before the Tertiary period. Birds occur first in 

 the Upper Jurassic, but both on this horizon and in the 

 Cretaceous they retain conspicuous characters of their ancestry 

 which have subsequently disappeared ; they seem to have 

 become dominant contemporaneously with the mammals at the 

 beginning of the Tertiary period. 



Range in Time. Gradual evolution whether in the form 

 of progression, retrogression, or differentiation is usually ob- 

 servable even in the minor division's when their range can be 

 traced through the geological formations ; and characters change 

 more or less slowly in proportion to their magnitude. In all 

 satisfactorily known instances, an order exhibits a longer 

 geological range than any of its contained families ; its family- 

 types persist for a longer time than any of the genera grouped 

 under them ; while the genera themselves remain for a more 

 extended period than the species. A highly specialized 

 member of any division is also more liable to early extinction 

 than its more generalized congeners, probably from its less 

 adaptability to changes in the environment. An illustrative 

 case may be cited. The order Ungulata (hoofed-mammals) is 

 known to range from the very earliest Eocene strata to the 

 present day ; the family Equidse (horses), as commonly under- 

 stood, arises in the Upper Miocene ; the typical species of the 

 surviving genus Equus appear first in the Lower Pliocene. 

 One genus of this family (Hipparion), with highly complex 

 teeth, was restricted in its range to the Upper Miocene and 

 Lower Pliocene periods, while another (Hippidium), with much 

 specialized nasal region, had only a brief existence in South 

 America; whereas Equus itself with more normal teeth and 

 rostrum has survived from the Lower Pliocene to the present 

 day. In like manner, the highly specialized rhinoceros, Elasmo- 

 therium (Pleistocene of Russia), had a very limited range in 

 time and space compared with its more ordinary allies. 



Persistent Types. There are a few noteworthy exceptions 

 to the common rule last mentioned, which still await ex- 

 planation. These are generally referred to as "persistent 



