CHANGES OF PROPORTION 271 



in the reduction of the number of the pairs of grinding teeth, 

 from seven to six and finally in the adult modern elephant 

 stage to one. The addition of new characters is principally 

 observed in the remarkable evolution of the plates of the grind- 

 ing teeth and of the elaborate muscular system of the pro- 

 boscis. It is very important to note that, as in the evolution 

 of the horses (p. 263), this evolution independently follows sim- 

 ilar lines among the Proboscidea throughout all parts of the 

 world. In other words, the unity of the evolution of the 

 proboscidians in various parts of the world was not main- 



- 3» j^, 



Fig. 133. Groups of Reindeer (Ra>i(>ifci- taraudus) and Woolly Mammoth {Elcphas 



primi genius). 



Conditions of the reindeer-mammoth period of Europe during the maximum cold of the 

 fourth glaciation of the Glacial Epoch. Mural painting in the American Museum of 

 Natural History, painted by Charles R. Knight, under the direction of the author. 



tained by interbreeding^ but by the unity of ancestral heredity 

 and the unity of the actions, reactions, and interactions of 

 the animals with their environment. Widely separated de- 

 scendants of similar ancestors may evolve in a closely but not 

 entirely similar manner. The resemblances are due to the 

 independent gain of similar new characters and loss of old 

 characters. The differences are chiefly due to the unequal ve- 

 locity of characters; in some lines certain characters appear or 

 disappear more rapidly than others. 



The general fact that the slow-breeding elephants evolved 

 very much more rapidly than the frequently breeding rodents, 

 such as the mice and rats (Muridae), is one of the many evi- 

 dences that the rate of evolution may not be governed by the 

 frequency of natural selection and elimination. For example, 



