7O EXTINCT ANIMALS. 
depending largely upon the imagination and ingenuity of 
the reader. 
Lastly, we may note that there are important animal 
types which have their origin, and their end, within the 
geological record. These are called extinct animals and their 
study forms one of the most interesting chapters in zoology.* 
Orders and classes which now are represented by a small 
remnant ajypear to have flourished in the past in an 
astonishing manner until they were replaced by other types. 
The proximate agent causing their extinction may in many 
cases be obscure, but it is evidently part of a general law 
which ensures that the phylum, the class, the order, the 
genus, and the species shall arise, flourish, and depart in 
the same way as the individual. 
* The study of extinct animals is often termed Paleontology, but it 
is inseparable from Zoology. 
