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THE AMERICAN NATURALIST [Vol. XLV 



tions, discontinuity is the characteristic feature in the 

 change of type. 



As to populations, the biotypes of which may practi- 

 cally exhibit continuous transitions— like the case of my 

 own populations of beans — the idea might be born that 

 biotypes are evolved from each other by extremely small 

 steps in genotypical change. Hence such mutations must 

 be practically identical with "continuous" evolution. 

 But there is no evidence for this view. Certainly in such 

 populations the "static" transitions between the geno- 

 typical differences manifesting themselves in several 

 characters may be called continuous— but such a "con- 

 tinuity of museums," as it might be called, is not at all 

 identical with genetic continuity. Galton himself has 

 emphasized the capital difference between the notions of 

 continuity in collections and continuity in origin, and as 

 yet the mutations really observed in nature have all 

 shown themselves as considerable, discontinuous salta- 

 tions. So in my own still unpublished experiments with 

 pure lines. Natura facit salt us. The chemical analog 

 to such mutations may be the formation of homologous 

 alcohols, acids and so on. The greater imitations may be 

 symbolized by more complicated molecular alterations. 

 But such analogs are of very little value for the under- 

 standing of genetic evolution. 



The genotype-conception supported by the great stock 

 of experiments as to pure line work, Mendelism and muta- 

 tions does not consider personal adaptation as a factor of 

 any genetic importance. Phrases as "characters, won by 

 adaptation and having successively been hereditarily 

 fixed," are without meaning from our point of view. 

 Hence much talk of adaptive characters successively 

 gained seems to us an idle matter. A closer study of 

 desert-organisms and the like may elucidate such things; 

 here the suggestive researches of Llovd as to stomates in 

 desert plants may be pointed out. And as to the old 

 question of "mimicry," this problem in the famous cases 

 of butterflies has in a most convincing manner been put 



