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THE AMEBIC AN NATURALIST [Vol. XLIV 



A word more on certain general questions. Can we 

 conclude that if selection lias no dynamic effect in chang- 

 ing existing genotypes, that therefore it need not be 

 reckoned with in evolution? Or must we conclude that 

 if it is to be reckoned with at all, selection has oppor- 

 tunity to act only on large leaps in evolution ; that evolu- 

 tion takes place by such leaps, and not by imperceptibly 

 small changes? 



Such evidence as the pure line work gives implies 

 neither of these things. The differences between the 

 diverse pure lines hurc arisen in some way, if evolution 

 occurs, and once these differences have arisen, they are 

 open to the operation of selection as are any other dif- 

 ferences. What the pure line work shows (agreeing 

 in this with other lines of evidence) is that the changes 

 on which selection may act are few and far between, 

 instead of abundant; that they are found not oftener 

 than in one individual in ten thousand, instead of being 

 exhibited on comparing any two specimens; that a large 

 share of the differences between individuals are not of 

 significance for selection or evolution— these being pre- 

 cisely the differences measured as a rule by the bio- 

 metrician's "coefficient of variation." Thus the work 

 of natural selection is made infinitely more difficult and 

 slow; but logically it is still possible. 



Nor does the pure line work assist natural selection, 

 as some have hoped from the mutation work, by making 

 the steps in evolution greater in amount. On the con- 

 trary, the work witli genotype- brings out as never be- 

 fore the minuteness of the hereditary differences that 

 separate the various lines. These differences are the 

 smallest that can possibly be detected by refined meas- 

 urements taken in connection with statistical treatment. 

 Johannsen found his genotypes of beans differing con- 

 stantly merely by weights of two or three hundredths 

 of a gram in the average weight of the seed. Genotypes 



of heredity. The cases of Castle and of Bateson will occur to every 



