cook: evolution through normal diversity 193 



The statement and its context are significant, and the issue 

 is fundamental, though obscured by much deductive reasoning. 

 Inheritance of "acquired characters," the changes that are 

 directly imposed or induced by the environment, is no longer 

 credited, but the environment is still supposed to cause evolu- 

 tionary changes indirectly, through the medium of selection. 

 It has been argued in many ways that evolution must be due 

 to environmental causes, but such inferences do not visualize 

 against a biological background. Not only is current dialectic 

 unclear in failing to recognize elementary distinctions between 

 causal and conditional relations, but essential facts continue 

 to be neglected and dangerous applications entertained, such 

 as the theory of natural selection used in Germany to justify 

 the war. 



Historical interest may be claimed for the passage from Mee- 

 han on account of the definite recognition of normal diversity. 

 The expression "power to vary" may be misunderstood as 

 assuming a mysterious "principle" or hidden "mechanism of 

 evolution," but evidently it refers to the concrete, visible fact 

 of diversity as the general and normal condition among the mem- 

 bers of species. "The variations must be from some natural 

 law of evolution inherent in the plant itself" was an earlier 

 expression of the same idea that normal evolutionary diversity 

 is a general fact, not determined by environment.^ 



Darwin recognized variation as a general fact, but he was 

 wont to consider the environment as the cause, and the belief 

 in environmental causation became completely dominant in the 

 minds of many of his followers. Meehan's conception of varia- 

 tion as independent of the environment was framed many years 

 in advance of the formal recognition of heterism or normal di- 

 versity among the members of species as a general evolutionary 

 fact. Owing to the preponderance gained by the Darwinian 

 interpretation, the word variation had come with many writers 

 to refer almost exclusively to differences of accommodation to 

 environmental conditions. The new word heterism seemed 



2 T. MbEhan. On the agency of insects in obstructing evolution. Proc. Acad. 

 Phila. 1872: 237. 



