408 EVIDENCES OF EVOLUTION 



lief is as convincing as that underlying the phenomena 

 of gravitation and chemical affinity. As indicated in the 

 foregoing paragraph, the study of paleontology, or fossil 

 remains in the crust of the earth, is a most important 

 source of information, but — contrary to the popular 

 belief — it is by no means the only one. In addition to 

 the proof furnished by (1) Paleontology, a wealth of evi- 

 dence has been supplied by studies in (2) Classification, 

 (3) Geographical Distribution, (4) Comparative Anat- 

 omy, (5) Embryology, (6) Artificial Selection, and (7) 

 Genetics. These topics will now be considered in some 

 detail. 



EvmENCE FROM PALEONTOLOGY. — It is oftcn SUppOSCd 



that if all the fossils in the earth's crust could be 

 studied, a fairly complete history of life could be written. 

 Such, however, is not the case. All the animals and 

 plants from the first appearance of living things down to 

 perhaps fifty million years ago have disappeared com- 

 pletely. If they ever were preserved as fossils, these have 

 been destroyed by heat, water, or other agencies. What 

 exists at the present day is only a small fraction of 

 the whole. Nevertheless, this remnant is highly illuminat- 

 ing and furnishes exactly those facts which are to be ex- 

 pected as a result of evolution. 



Some of the best known and most striking examples of 

 a long continued evolutionary history have been dis- 

 covered in the United States. Approximately three mil- 

 lion years ago, it is estimated, the climate of the country 

 was similar to that in Africa at the present time. Tropi- 

 cal vegetation extended as far northward as the Canadian 

 border, crocodiles inhabited the extensive marshes, while 

 herds of camels, elephants, and rhinoceroses roamed the 

 land. As time passed the temperature grarlually lowered, 

 the country became dryer, and large forested districts were 

 changed into treeless plains. Amid these shifting con- 

 ditions many animals and plants became extinct or mi- 

 grated into more favorable regions. Others underwent 



