THE JURASSIC PERIOD. 95 



phenomenon which grows more common as the deposits become more 

 recent, but is especially characteristic of stages of base-level and advan- 

 cing seas. It is made the more interesting by the presence, in the 

 same beds, of many land insects that suffered a similar fate. Not a 

 few of them were wood-eating beetles, thus giving a hint of the nature 

 of the battle of life, implying that the plants found enemies not only 

 in wind and storm, but in predaceous foes without and within. In 

 the closing stages, the land was extended, which should in itself have 

 been favorable to an expansional development of plants, but such 

 extensions of the land are so liable to be attended by adverse climatic 

 and topographic changes, that no safe inferences can be drawn except 

 from the actual record, which is rather scanty. In the heart of the 

 period, the distribution of genera and even of species was wide, both 

 in longitude and latitude, implying uniformity of conditions. Some 

 tendency to provincial limitation appears, as in the apparent restric- 

 tion of Ptilophyllum to India, Gingkodium to Japan, and the Abietinc 

 to northern Eurasia. The last has been made a basis for the suggestion 

 that a climatic differentiation had begun by the cooling of the northern 

 regions, a suggestion based on the assumption of a universal warm 

 climate in early times, sequent on a molten globe. The flora should 

 probably rather be interpreted as indicating that the period was one 

 of the series of periods marked by the mild, uniform climates attend- 

 ing base-level conditions and sea extension, which alternated with 

 periods of more diversified and occasionally severe climates. 



II. The Land Animals. 



Classificatory difficulties. — The discussion of the land animals of 

 the Jurassic Period is embarrassed by a systematic infelicity in the 

 accepted methods of limiting " Periods." Technically, periods are 

 founded essentially on marine formations and marine life ; and properly 

 so, because these have given by far the best record, and most closely 

 reflect the deformative movements that lie back of life changes. An 

 ideal marine period consists of a great advance of the sea upon the 

 continent, attended by an expansional evolution of the shallow- water 

 life, followed by a withdrawal of the sea, attended by a restrictional 

 evolution of the life. The ideal division between such periods is obvi- 

 ously the time of maximum withdrawal, when the fauna developed in 

 the expansional stage is being reduced to its lowest terms by restriction, 



