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THE AMERICAN NATURALIST [Vol. LI 



fact, on certain phases circumstantial evidence is the only 

 evidence at hand. 



The exact nature of the cause or causes of bud-variation 

 can hardly be discussed profitably. We may imagine 

 irregularities of cell division directed by combinations of 

 unknown factors, but to describe these factors in concrete 

 terms is at present impossible. At the same time, cause 

 can not be neglected entirely even at present, for cause in 

 a generalized sense is intimately connected with frequency 

 in that vigorous perennial the question of the inheritance 

 of acquired characters. The data on this subject are so 

 voluminous that each for himself must give them careful 

 conscientious consideration. Here no more can be done 

 than to point out some of the conclusions to which I, per- 

 sonally, have been driven, and their connection with the 

 subject in hand. These conclusions are: 



1. Broad and varied circumstantial evidence indicates 

 unmistakably that the inheritance of acquired characters 

 has played an extremely important role in evolution. 



2. Numerous experimental investigations designed to 

 test the possibility of such inheritance directly have either 

 failed utterly or have been open to serious destructive 

 criticism. Direct proof of the inheritance of acquired 

 characters is therefore lacking. 



3. If conclusions 1 and 2 are to be harmonized, either 

 modifications are fully inherited so rarely that proof that 

 they do not belong to the general category of chance 

 changes in constitution of the germ-plasm is impossible, 

 or the imprint of the environment is so weak that ex- 

 tremely long periods of time— perhaps geological epochs 

 —are necessary for its manifestation. 



Diametrically opposed views on the inheritance of ac- 

 quired characters are held tenaciously and unequivocally 

 by equally eminent biologists. Those who concur with 

 the Laniarckian position are nearly always the students 

 of evolution who approach the subject from the historical 

 or the philosophical side and who rely almost entirely 

 on circumstantial evidence ; those who adhere to the side 



