474 GENERAL CONSIDERATIONS 



in a way, related to homology and analogy are those of convergence and 

 divergence. As a result of convergence, which occurs when dissimilar 

 animals become adjusted to a common environment, animals very unlike 

 in ancestry and different in structure have a close superficial resemblance 



Fig. 3.54. — Figures to illustrate convergence. A, a mackerel-shark. (Based upon 

 Jordan, "Guide to the Study of Fishes.") B, restoration of a fossil aquatic reptile, Ichthyo- 

 saurus. (Based upon Schuchert, "Historical Geology.") C, a dolphin. (Based upon 

 Brehm, " Thierleben.") 



to each other, while as a result of divergence related animals, because of 

 adaptation to dissimilar environments, become quite unlike. Con- 

 vergence is well illustrated by aquatic types belonging to different classes 

 of vertebrates, which are similar in the shape of their bodies and the 

 character of their locomotor organs (Fig. 354) . Divergence is shown very 

 strikingly by mammals, which, derived from a cursorial ancestor, have 

 become modified for fossorial, arboreal, volant, and aquatic life. 



