502 



THE THEORY OF EVOLUTION 



with an ideal type, hence the homologies. This was the belief 

 of Louis Agassiz (1807-1873), who was unable to adjust his think- 

 ing to the concept of evolution as set forth by Darwin in 1859. 

 Such an idea is a theoretical possibility, provided there is evidence 

 that animals originated in their present form and have not changed 

 since their appearance on the earth. However, species do not 

 seem to have originated in their present form and they do seem to 



Fig. 265. — Representative fossil records. 



Left, fossil tree trunks; right, section of Amythest Mountain, Yellowstone National 

 Park, showing by fossil tree stumps that at least seventeen forests became established 

 and were successively killed out by deposits of volcanic ash. Some two thousand feet 

 of rock are shown, and the whole rests upon other strata that show unconformity due to 

 erosion of earlier land surfaces (cf. Fig. 274). (Left, photo, by courtesy of the American 

 Museum of Natural History; right, after W. H. Holmes, photo, by courtesy of H. F. Cleland.) 



have changed during geologic time. The evolutionary explana- 

 tion of these anatomical resemblances between animals is, of 

 course, that the members of any group, as the vertebrates, for 

 instance, are similar in structure because they have inherited a 

 similar plan of organization from ancestors which all vertebrates 

 had in common. Each subdivision and species has been modified 

 in particular ways in relation to its habits of life. They are fun- 

 damentally alike, however, because they have never lost the under- 

 lying plan of body that existed in their ancestors. Because whales 



