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THE AMEBIC. I .V \\ I TUB. I LIST [Vol. LIT 



a quantitative character even when additional mutations 

 and their recombinations make them at first sight appear 

 diversified. This uniformity indicates the adaptation^ 

 type produced by selection of the quantitative variations 

 of some vital gene ; the differences are only a difference 

 in apparel. 



In conclusion, we may point out three groups of facts 

 which, of the greatest importance for evolution, have 

 always been a hard nut for the mutationists to crack. 

 The first is the series of temperature-experiments in Lepi- 

 doptera— and similar experiments in Amphibia, Crus- 

 tacea, etc.— that lead to the production of aberrant forms 

 which resemble closely certain geographic varieties. But, 

 with the exception of certain often-quoted cases, these 

 aberrations are not hereditary. In the light of our experi- 

 ments these facts are not surprising. The effect of the 

 temperature experiments is to change the normal time- 

 curve of certain metabolic processes. The effect is, there- 

 fore, due to this change of one of the variables of the reac- 

 tions in question. The quantitative change of the sub- 

 stance of a gene, however, which we found to be at the 

 basis of the geographical variations, also produces a dif- 

 ference in respect to the time-curve and therefore the 

 same effect, this time a heritable effect. If we now select 

 the plus individuals in this type of experiment— and this 

 applies to all analogous experiments— we may simply se- 

 lect a modification. But we also may select the combina- 

 tion of a plus-modification with a plus quantity of the 

 gene in question. If the experiment is repeated, the next 

 generation will then show a still stronger reaction, or, if 

 the experimental influence is not repeated, there will 'be 

 an after effect of the experiment on the parents. It is 

 remarkable that such results, which were to have proved 

 the inheritance of acquired characters, always turned out, 

 when characters relating generally to the life-cycle were 

 in question, characters which also appear in the geo- 

 graphic races of the form. Extreme mutationists used to 

 deny or disregard these facts. Here we have a simple 



