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THE AMERICAN NATURALIST [Vol. XL 



which, after a period, grows weaker and weaker, so the duration 

 of the other may be governed by tlie quantity of proHfic power 

 bestowed upon the species which, after a season, may decline in 

 energy, so that the fec-uiidity and multipHcation of in(hviduals 



fatal term arrives when the embryo, incapable of extending and 

 developing itself, abandons, almost at the instant of its formation, 

 the slender principle of life by which it was scarcely animated, 

 and so all dies with it." ^ Lyell opposed this doctrine on the ground 

 that there is seldom evidence of physiological deterioration in the 

 last representatives of a species. 



This idea of self extinction, as applied to a theoretical arrest of 

 variation was expressed in another form by Darwin and Wallace 

 and has been recently revived. 



The Limiting of Variation. — The theoretical importance as- 

 signed to the limiting of variation (independently of environment) 

 as a cause of extinction depends partly upon one's theoretical 

 opinions as to the modes of evolution. In the citations made above 

 from Darwin (p. 852), Wallace (p. 852) and Andrews (p. 853) the 

 theoretical view is taken that since (1) a limitation or cessation of 

 fortuitous variation would cut off material for improvement 

 through selection, (2) a fixed or non-adaptable type would arise and 

 (3) extinction would follow. 



Similarly Mr. C. B. Crampton (as cited by C. W. Andrews)^ 

 suggests an inherent cause of extinction as follows: " .... In a 

 recent paper by Mr. C. B. Crampton (Proc. Roy. Phys. Soc. 

 Edinburgh, vol. xiv, p. 401) a possible inherent cause of extinction 

 is suggested. It is impossible to do justice to this interesting 

 paper in a short note, but the gist of the argument seems to 

 be as follows: — In the original unicellular organism the possi- 

 bilities of variation are almost infinite, but as soon as evoli.tion 

 along any line begins, these possibilities are restricted, and IxMoine 

 more and more so the more highly specialized the animal is; in 

 short, the potential variation of an organism becomes less and U\ss 

 as specialization advances. Furthermore, under the influence of 



> Lyell, Charles. Principles of Geology, vol. 2, 11th ed., 1892, p. 270. 

 * Andrews, C. W. "Some Suggestions on Extinction." Geol. Mag., dec. 

 4, vol. 10, no. 463, p. 1, January, 1903. 



