136 THE SHORTER SCIENTIFIC PAPERS 



To these propositions may be added the suggestion of 

 the fundamental importance which physico-chemical 

 methods must play during the future in solving the prob- 

 lems of evolution. 



II. 



The controversies relating to evolution have been many. 

 When, however, one considers the interest attached to the 

 subject, its broad bearing on various phases of human 

 welfare — sociology and economics in general, animal and 

 plant breeding in particular — together with the difficulties 

 of interpretation which apparently have increased rather 

 than diminished during the sixty or more years seriously 

 devoted to its elucidation, it is not at all surprising that 

 many different conclusions have been reached, many 

 dogmatic statements presented, and many acrimonious 

 discussions engendered. 



In connection with a clearer understanding of the 

 points at issue, it will be well to pass certain historical 

 details relating to the development of the different 

 theories somewhat critically in review. This is done even 

 at the risk of a repetition of facts quite familiar to those 

 who have taken more than a passing interest in the subject. 



For long the theory of natural selection dependent on 

 the inheritance of small chance variations received gen- 

 eral acceptance. Championed by Weismann in his notable 

 controversy with Spencer to the exclusion of the Lamarck- 

 ian idea that characters acquired through environmental 

 stimuli were heritable, it seemed at the time entirely 

 plausible as an explanation meeting the conditions. 



With the greater attention given to experimental meth- 

 ods, however, doubt arose concerning the fundamental 

 value of selection and resulted in the presentation of the 

 mutation theory by DeVries. Here evolution was inter- 

 preted as arising from sudden and comparatively extreme 

 variations passed on by inheritance in nearly an unchanged 

 condition. Once more the results of experimental work 

 along the lines of the rediscovered principle of Mendelian 

 segregation indicated to a large number of students of 

 evolution that the facts set forth by DeVries were subject 

 to quite another explanation, in itself having no bearing 



