GENERAL PRINCIPLES 3 



succeeding epochs higher and more complex types were gradually 

 developed until the highly organized forms of the present time, 

 culminating in man, were produced. It should be remembered, 

 however, that not all change in organisms has been progressive, 

 but rather only a single thread. 



4. No species once extinct has ever reappeared. Numerous 

 important species have lived through many epochs of geologic 

 time, while others have had only brief existence. In no case, 

 however, has a species once become extinct been known to re- 

 appear. 



5. While higher and higher types have been developed during 

 geologic time, many of the earlier and simpler types have persisted. 

 Thus Foraminifers, which are exceedingly simple, single-celled 

 animals, have lived in the sea from early geologic time to the 

 present. 



6. The broader or larger biological groups of organisms have 

 persisted longer than the smaller. No subkingdom has ever become 

 extinct, though species frequently have not outlived even a single 

 geological epoch. As a rule, genera have survived longer than 

 species, orders longer than genera, etc. 



7. The life history of the individual tends to recapitulate the 

 evolution or history of the race. A Frog, which is a typical Am- 

 phibian, shows certain fish-like characters during its embryonic de- 

 velopment, as, for example, the presence of gills and tail. Again, 

 the modern Crab, which is a Crustacean, shows a gradual shorten- 

 ing of the tail portion during its embryonic development. The 

 earliest known Crustaceans were practically all long tailed. 



Fossils and their Significance 



Traces or remains of plants and animals preserved in the rocks 

 are known as fossils. The term originally referred to anything 

 dug out of the earth, whether organic or inorganic, but for many 

 years it has been strictly applied to organisms. Paleontology, 

 which literally means "science of ancient life/' deals primarily 

 with fossils. 



Darwin thought that the stratified rocks contain only a very 

 incomplete record of the geologic history of life. Though many 

 thousands of species of fossils have been described from rocks of all 

 ages except the very oldest, and more are constantly being brought 



