396 ELEMENTS OF BIOLOGY 



any country is derived from the history of the nation. The theories 

 concerning the mechanism of evolution are at present but sugges- 

 tions which provoke thought and stimulate investigation. But the 

 theories are of themselves indecisive. It has come to be recognized 

 more and more that a final explanation rests upon a more complete 

 understanding of the whole nature of the physical and chemical 

 processes that underlie vital phenomena. Science attains progress 

 by adopting the mechanistic point of view as a working hypothesis. 

 To be consistent in his science the biologist must also take the view 

 that the stupendous pattern of evolution constitutes orderly changes 

 in the expression of the energy transformations in protoplasm over 

 an unbelievably long period of time. We do not yet know exactly 

 how these transformations take place in the most simple type of 

 cell carrying on its most simple vital characteristic. Consequently as 

 modern biologists concern themselves more and more with rigorous 

 examinations of the physical and chemical activities of protoplasm, 

 the controversies over the probable method of evolution recede 

 more and more into the background. Evolution is taken as an es- 

 tablished fact which makes rational and unifies the facts of anatomy, 

 of embryology, and of many other aspects of the organism. 



The Evidence from Paleontology. The evidence that pres- 

 ent-day complex plants and animals, including Man, have evolved 

 in long past ages from more simple and more primitive forms, has 

 accumulated from a variety of sources. 



Life has existed on the earth for millions of years. During this 

 time many types of animals and plants that are not to be found 

 today have appeared, lived through periods of perhaps hundreds of 

 thousands of years, and then have become extinct. The study of the 

 records of plant and animal life in the form of fossils, consisting of 

 flinty replacements of their bodies, of footprints, of imprints of 

 their forms in rocks, and of shells and actual skeletons, constitutes 

 the fascinating subject of paleontology. Paleozoology deals with 



