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. 
