38 EVOLUTION AND GENETICS 



of one individual, is a constant repetition of relationships of species 

 with species and group with group. It is impossible to go into the 

 minor relationships in such a work as this, but in the major points 

 of similarity mentioned in tliis chapter we have an outline which 

 greater elaboration merely amplifies. 



Summary. The time involved in evolution is so vast that our 

 observations of organisms cover, in proportion, only a moment. 

 Nevertheless our records contain evidence of relationship. These 

 are expressed in the classification of organisms into species and 

 the association of species into successive groups of gradually 

 increasing scope. Within this system relationship is evident be- 

 cause of similarities of structure, of which protoplasm and the cell 

 are common to all organisms. Among the animals a definite plan 

 of structure leads to a secondary grouping of the various phyla. 

 Relationship of the various groups is based on the fact that they 

 always present some expression or modification of the funda- 

 mental plan, however different they may be in details. 



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Montgomery, T. H., Proc. Acad. Nat. Sci. Phil, 1902. 



Powers, E. B., Am. Nat. XLIII, 1909. 



Coulter, J. M., Barnes, C. R., and Cowles, H. C, Textbook of Botany, 1910. 



Parker, T. J., and Haswell, W. A., Textbook of Zoology, 3rd edition, 1922. 



Thomson, J. A., Outline of Zoology, 6th edition, 1914. 



LiNDSEY, A. W., Denison U. Bulletin, Jn. Sci. Lab. XX, 289-305, 1924. 



Hegner, R. W., College Zoology, revised edition, 1926. 



Woodruff, L. L., Foundations of Biology, 3rd edition, 1927. 



