[Chap. XL VARIATIONS AND DIVERSITY OF ORGANISMS 503 



their gene complements were largely alike but somewhat unlike. If herit- 

 able variations are dependent primarily upon intrinsic conditions, there 

 should be both parallel and divergent evolution, regardless of whether 

 the environments in the isolated areas are similar or dissimilar. 



In the chapter on plants of the past, we shall see that various kinds of 

 plants evolved, flourished for a time, and then became extinct. The 

 redwoods, ginkgo, and stinking cedar as well as numerous other species 

 of plants that were once widely distributed became extinct everywhere 

 except in a few local areas. Yet they grow well today when planted in 

 other parts of the world. They do not reproduce naturally in all these 

 new environments. Within the first third of the present centurv the 

 American chestnut was almost exterminated by a parasitic fungus. Other 

 species of plants are being diminished by insects, bacteria, viruses, and 

 fungi. By destroying natural habitats, man is also exterminating species. 

 Similar phenomena also occur in the animal kingdom. Toward the close 

 of the glacial period several kinds of animals became extinct on the 

 American continent. Among them were species of elephant, horse, camel, 

 rhinoceros, and tiger. The causes of their failure to survive are not 

 known, but among probable eliminating factors are diseases, changes in 

 physical environment, failure to reproduce, overadaptation, and perhaps 

 the accumulation of lethal genes. 



REFERENCES 



Berry, E. W. Tree Ancestors. Williams & Wilkins Company. 1923. 



Darwin, Charles. The Origin of Species. A. L. Burt Company, Inc. 1859. 



De\^ries, Hugo. Species and Varieties: Their Origin by Mutation. Open Court 

 Publishing Company. 1912. 



Dunn, L. C. Hereditij and Variation. University Society. 1931. 



Gruenberg, B. C. The Story of Evohition. D. Van Nostrand Company. 1929. 



Hiesey, W. H. Environmental influences and transplant experiments. Bot. Rev. 

 6:181-203. 1940. 



Hurst, C. C. The Mechanism of Creative Evolution. The Macmillan Company. 

 1932. ( For additional illustrations ) 



McAtee, W. L. Survival of the ordinary. Quar. Rev. Biol. 12:47-64. 1937. 



Newman, H. H. Evolution Yesterday and Today. Williams & Wilkins Com- 

 pany. 1932. 



Oparin, A. I. TJie Origin of Life. The Macmillan Company. 1938. 



Schaffner, J. H. Color in various plant structures and the so-called principle 

 of selective adaptation. Ohio Jour. Sci. 23:182-191. 1933. 



Smith, H. W. Kamongo. The Viking Press. 1932. 



