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SCIENCE 



[N. S. Vol. XXXIX. No. 1005 



organism which will continue to transmit 

 its characters ? The answer must be in the 

 negative so far as the originating of any- 

 thing new is actually concerned. Recessives 

 may be obtained. Characters may be re- 

 distributed. They were present in the 

 forms first utilized, however. 



The mutation theory formulated by De 

 Vries in 1901 approximately at the time in- 

 terest was being awakened by the rediscovery 

 of the hybridization principles of Mendel, 

 needs no extended explanation to those who 

 have been interested in evolution. Based 

 on cultural experiments with (Enotliera 

 lamarckiana, one of the evening primroses, 

 the appearance of relatively small numbers 

 of forms which were quite distinct from the 

 parental species and which bred true in 

 subsequent generations, led to the inference 

 that evolution had in many cases proceeded 

 by discontinuous variations or mutations. 



Long series of breeding experiments fol- 

 lowed in connection with other organisms, 

 both plants and animals, with results quite 

 similar to those obtained by De Vries. In- 

 vestigations were also made (Fischer, Mac- 

 Dougal, Tower, etc.) where organisms were 

 subjected to stimuli abnormal in their 

 nature, with the result that a modified prog- 

 eny was obtained which bred true to the 

 apparently induced character in succeed- 

 ing generations. Furthermore, cytological 

 studies (Gates, etc.) demonstrated some 

 interesting relationships so far as differ- 

 " mutants" were concerned. 



While the evidence is far too insufficient 

 to allow more than a tentative opinion, 

 there are several conclusions concerning 

 mutation which appear justified. The na- 

 ture of the results obtained through the 

 various agencies make it quite evident that 

 they are not all due to a single underlying 

 principle. There are many "mutants" the 

 origin of which is most certainly to be 

 explained on the basis of a heterozygous 



condition of the gametes, and much evi- 

 dence has accumulated that 0. lamarckiana 

 of De Vries on which the mutation theory 

 was founded belongs to this class. Further- 

 more there are mutants developing in con- 

 nection with the action of abnormal stimuli 

 although it is not at all improbable that 

 some of these result from heterozygotes. It 

 may be mentioned that Humbert (1911) 

 in experiments with 7,500 pure line plants 

 of Silene nocti flora, one of the "pinks" 

 utilizing methods similar to those of Mac- 

 Dougal, failed to obtain any "mutants." 

 Another explanation of the results in con- 

 nection with the influence of abnormal 

 stimuli is that the modification takes place 

 through the destruction of a factor and thus 

 the process is one of subtraction instead of 

 addition. There are also investigations, 

 notably those of Gates, in which the aber- 

 rant organism apparently results from the 

 abnormal behavior of the chromosomes at 

 some stage during the life cycle. (Enothera 

 gigas with its tetraploid chromosomes is 

 here of much interest. 



Notwithstanding these diverse results, 

 there is little indication that anything 

 actually new has been added to the organ- 

 ism which would not have occurred within 

 a pure line. If this is true the heterogene- 

 ous school of mutationists can be of little 

 assistance beyond suggesting the way in 

 which evolution did not take place. 



The experiments on the basis of pure 

 line breeding belong to a comparatively 

 recent period and are of the utmost im- 

 portance. Johannsen in 1903 published re- 

 sults based on a pure line of beans self- 

 fertilized for successive generations and 

 evidently homozygous. From a bean 

 weighing 95 centigrams and far above the 

 average in size he obtained plants produc- 

 ing beans varying in weight from approxi- 

 mately 35 to 70 centigrams, but all far 

 below the weight of the parent. Utilizing 



