340 GENERAL ZOOLOGY 



later replaced by one-toed types, and in the Pliocene 

 appeared the genus Equus (J, K) to which all living horses, 

 asses, and zebras belong. 



GENERAL REMARKS ON PAL^OZOOLOGY 



The geological periods and the important features of 

 palseozoology are shown in Fig. 113. There is no doubt 

 that the great landmarks of this history have been correctly 

 interpreted strata of sedimentary deposits have been 

 formed on the earth's crust and the deepest of these are 

 the oldest. From studies of the characteristics of rocks 

 and the animal remains in such deposits the following gen- 

 eralizations may be made : 



1. Conditions on the earth have varied. There have been 

 times of rapid change when whole groups of animals became 

 extinct because they were unsuited to new conditions, and 

 these have alternated with long periods of comparative 

 stability when particular types of animal life multiplied 

 and became specialized along various lines. 



2. The animals of the earliest times were predominantly 

 marine and in later ages the land and fresh water have had 

 an increasing population. Structure is so correlated with 

 habits of life, that, taken with the nature of the sediment 

 in which fossils are found, it enables a palaeontologist to 

 picture the conditions in which an animal lived millions 

 of years ago. 



3. The geographic distribution of animals today is corre- 

 lated with their origin and distribution in the past. A group 

 of limited range is usually of rather recent origin, whereas 

 one which is widely distributed is old. Zittel says, "An 

 understanding of the physical conditions which have gov- 

 erned the perpetuation of recent plants and animals in their 

 respective provinces would be utterly impossible without a 

 knowledge of their distribution in former times." 



4. The simplest, or most primitive, animals came first. 

 This is equally applicable to the sequence in the develop- 



