SEA-SERPENTS. 147 



geologist. The spur was small and of soft material, and we 

 speedily removed it in blocks, to the level of the reptile, and 

 took out the remains as they lay across the base from side to 

 side." 



In taking leave of the " Age of Reptiles," we cannot but 

 marvel greatly at the diversity of forms assumed by the various 

 orders of this class, their strange uncouth appearance, their 

 assumption, in some cases, of characters only known at the 

 present day among the mammals, their great abundance, and the 

 perfect state in which their remains have been preserved in 

 the stratified rocks of various parts of the world. And the 

 reader may naturally ask, " How is it that so many types have 

 disappeared altogether, leaving us_ out of a total of at least nine 

 orders, only four, viz. those represented by crocodiles, lizards, 

 snakes, and turtles ? " To such a question we can only answer 

 that the causes of the extinction of plants and animals in the 

 past are not yet known. Climate, geographical conditions, food- 

 supply, competition, with other causes, doubtless operated then as 

 now ; but if there is one clear lesson taught by the record of the 

 rocks, it is this that there has been at work from the earliest 

 periods a Law of Progress, so that higher types, coming in at 

 certain stages, have ousted the lower types, sometimes only 

 partially, sometimes completely. But why the Dinosaurs, for 

 instance, perished entirely, while the crocodiles survived to the 

 present day, no one can yet explain. We can see no reason, 

 however, why such problems as these should not be solved in the 

 future by the co-operating labours of naturalists and geologists. 



In the great onward and upward struggle for existence, higher 

 types have supplanted lower ones ; and, in accordance with this 

 biological truth, we find that in the next era (known as the 

 Tertiary or Cainozoic) the mammal held the field while the 

 reptile took a subordinate place. 



