210 THE AMERICAN NATURALIST [Vol. XLI 



gathered broadly; so that we may fairly ask the experimental 

 school to admit that results, however well proved for the condi- 

 tions established by the experimenter, ought to be assigned little 

 worth if they find only a trivial correspondence in nature at large. 

 We make the same demand of the physiologist with respect to 

 such a phenomenon as geotropism, for instance. The extended 

 study of this form of irritability has its justification only in the 

 fact that plants in nature so widely show the effects of geotropism 

 in their forms; the value of the experimental results is great 

 because the phenomenon is manifestly widespread in free nature, 

 being observable in the erect attitude of countless main axes 

 in field and forest, in the fixed angles of side stems, the vertical 

 descent of tap-roots, etc., etc. In like manner a true theory of 

 specific origins should find strong confirmation in the study of 

 the broadest aspects of plant and animal life. Every grand 

 agent of specific modification should leave its distinctive mark 

 upon the character of life as a whole, and if we rightly apprehend 

 the nature of the agent we may expect to be able to distinguish 

 its special mark or effect when we know plants and animals 

 thoroughly. I think that it will appear from considerations 

 which I now bring before you that the distribution of species 

 must have peculiarities corresponding to the particular class of 

 evolutionary forces which have been at work. If this be so, suit- 

 able studies in geographic taxonomy must possess high evalua- 

 tory worth when we wish to estimate theories of evolution. 



The Effects of Different Evolutionary Agencies upon 

 Specific Distribution 



Let us examine the necessary effects of the chief supposed 

 evolutionary agencies upon the character of specific distriluition; 

 and first contrast Natural Selection in I)ar\vin\ stricici- sense 

 with Mutation, in this regard. Natural Selection works within 

 specific limits. Its materials are the small, or individual, varia- 

 tions within the species. By the accumulation of tiiese variations 

 as they occur from generation to generation new characters are 

 built up. The change in a species is slow and the whole species 

 within a given competitive area moves along together. When 



