A VIEW IN PERSPECTIVE. 123 



ungulates. In short, many of the forms found in 

 these rocks are so intermediate in type between the 

 orders which we find in the world to-day that they 

 cannot be regarded as belonging to any of them, and 

 at the same time" they show so many characters in 

 common with the mammals now existing that it 

 is equally impossible to regard them as forming dis- 

 tinct orders. They were, in fact, intermediate types. 

 It is commonly true that while the fossils require 

 no new. types created for their classification, the 

 fossils representing the introduction of any group 

 show such a complication and combination of rela- 

 tions that it becomes impossible to classify them 

 satisfactorily into any of the modern orders. As 

 we study fossils of the succeeding rocks, however, 

 we find the similarity of the structure to the modern 

 orders more and more close, so that the nearer we 

 approach the present > time the easier becomes the 

 task of fossil classification. The interpretation of 

 this is of course found in the fact that the various 

 orders of animals have separated from common 

 centres in accordance with the principle of descent, 

 or otherwise. The nearer we come to those centres 

 the greater is the similarity of the animals ; it was 

 only in later times that the different lines of descent 

 became sufficiently separated from each other to be 

 recognized as distinct orders. 



Introduction^ Development, and Decline of Types. 



We must now notice more particularly the history 

 of some representative groups, studying their method 

 of introduction, expansion, and extinction. In gen- 



