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PART III— VARIATION. 

 The Study of Variation.* By C. H. Eigenmann. 



Variation and Its Importance. No two individuals are exactly alike. The 

 differences of whatever sort, whether in structure or habit, between the individ- 

 uals of a species, whether these individuals are related to each other as parent 

 and child, or belong to the same brood, are termed variation. 



The whole basis of the Darwinian idea of evolution is this individual vari- 

 ation. At present we have two estimates of the imjjortance of individual vari- 

 ation. 



I. The individual variations are of the utmost importance, and all species 

 are the result of natural selection working on the varying individuals of any 

 species. 



II. Individual "variation offers us little hope of learning the real facts of 

 evolution," "species are not the result of the selection of a few favorable vari- 

 ations out of a large number of haphazard changes," but to "the orderly ad- 

 vance (of the mean specific form) towards the final goal, deviating very little 

 from the direct line."t 



"We subscribe to neither of these views, wishing to view the facts as they are 

 presented by the conditions of the environment at Turkey Lake and the lakes in 

 the neighborhood, in a perfectly impartial way. 



The causes of variation are still unknown, though several explanations have 

 been attempted. This is not surprising since the variations in no s))ecies are 

 sufficiently known to formulate any satisfactory explanation, in fact little has 

 been attempted but to determine the extent of variation in comparativelv few 

 cases where the variation is great, resulting in the naming ot new varieties and in 

 the recording of abnormalities. The statistical method of studying variation is of 

 the most recent date, but much promises to be done with this method. 



Distribution of Variations. Variations are to be found at all times and 

 at all places where organisms exist. They are found under conditions where the 

 environment is in a state of stability. The conditions under which the greatest 

 variability is found liu fishes) are: 



1. Wide distribution. A large territory is, usually, though not necessarily, 

 inhabited by more or less stable varieties. 



'••■Contributions from the Zoological Laboratory of the Indiana I'niversity, Xo. 17. 



tThis wording is from Scott, but since the paragraphs are selected from isolated parts 

 of his paper, I do not wish to convey the idea that they state his views as he would like to 

 have them stated. The paragraphs state an extreme view. 



