334 



PRINCIPLES OF ANIMAL BIOLOGY 



represented in Eocene but long since extinct. This varied assemblage 

 appears suddenly in the earliest Cenozoic deposits, indicating a long 

 evolution before that era. The evolution of two mammals whose 

 histories are most completely preserved is presented later in another 

 connection. 



The purpose of the brief account in this section is to show the general 

 nature of the evolution of animals in relation to the evolution of the 

 earth's crust. So far as it relates to the vertebrate animals the story 



Fig. 280. — Skeleton of the armored dinosaur Stegosaurus. {From Lull, "Organic Evo- 

 lution," courtesy of The Macmillan Company.) 



is summarized by the diagram in Fig. 282. A similar chart for the more 

 numerous kinds of invertebrates would be too confused for our purpose. 

 Lines of Evolution. — Out of the wealth of fossil forms barel.y hinted 

 at above it is possible to select a few groups that show especially well 

 the step-by-step changes which animals have undergone. These gi'oups 

 are particularly instructive because the relative ages of their members 

 are not in doubt, and the differences between any two successive mem- 

 bers are so small as to leave no question that they possess genetic conti- 

 nuity. Such a scries of related forms is spoken of as a line of evolution. 



