i6 POPULAR SCIENCE MONTHLY. 



once by variation and by cross-fertilization. To what peculiarities of 

 substance or structure symbasis is due we have as yet no intimation, but 

 the same might have been said of gravitation and many other proper- 

 ties of matter for which names have proved useful, as well as of growth, 

 irritability, and similarly unexplained attributes of protoplasm. 



Variations do not appear and are not selected or accumulated merely 

 because of their usefulness or desirability with reference to environment, 

 but useless or even injurious characters may be adopted as a means of 

 evolutionary movement.* Specialization in the sense of extreme ac- 

 centuation of characters is often harmful and therefore not to be 

 ascribed to adaptation. The influence of natural selection increases 

 with the nicety of adjustment already attained, or as the range of 

 permissible variation is narrowed. Adaptive specializations also com- 

 monly imply a narrow dependence on external conditions, and thus 

 give no assurance of permanence for the type; they are more common 

 on the side-twigs of life than on the main branches. Evolution is both 

 accelerated and retarded by narrow selection or segregation ; accelerated 

 if the motion be estimated on the basis of a single character; retarded 

 if the organism be viewed as a whole. Normal evolutionary progress 

 does not go forward on the line of a single character, but requires the 

 accumulation of many variations to maintain the structural coordina- 

 tion and functional cooperation of parts. External modifications re- 

 quire less coordination than internal, and are often exaggerated far 

 beyond the requirements of use, and beyond the limits of develop- 

 mental welfare, f 



Organic change and diversity inside the species are necessary and 

 universal, but species and higher organic groups decline and become 

 extinct if their variations become limited to non functional parts and 

 do not provide, as it were, the facilities by which adjustment to chan- 

 ging environment may be maintained. Nevertheless, fitness for the 

 environment is only one aspect of the evolutionary problem; adapta- 

 tion is an incident and not a cause of evolutionary progress. Results 

 commonly ascribed to selection are due to the normal motion of organic 

 groups. Environment, including natural selection, segregation, isola- 



mutual advantage. Symbasis refers to the fact that organisms exist and make 

 normal evolutionary progress together or in groups commonly called species 

 rather than in simple or narrow lines of succession. 



* In Professor Baldwin's most recent and plausible improvement of the 

 static theory the preservation of new characters seems still to be ascribed 

 solely to natural selection. ('Development and Evolution,' p. 156, New York, 

 1902.) 



•f- 'The Origin and Significance of Spines: A Study in Evolution,' by Charles 

 Emerson Beecher, Am. Jour. 8ci., VI., 1-120, 125-136, 249-268, 329-359, 1898. 

 I am indebted to Mr. Charles Schuchert, of the U. S. National Museum, for 

 bringing this able paper to my attention. 



