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THE AMERICAN NATURALIST [Vol. LI 



tionary change. They have ranged from those who would 

 make the living world of to-day wholly the product of the 

 environment, to those that deny any participation of 

 external factors in the course of evolution. Where in 

 this array of opinion is the probable truth? To answer 

 this question will be to express another opinion; but it 

 is possible to formulate an opinion which is based upon 

 principle, and which will therefore be more inviting than 

 mere conjecture. 



With the aid of Sir Charles Lyell, who more than any 

 one else has taught us to seek the explanation of past 

 events in present processes, let us look about us for the 

 cause of diversity among individuals. We may ignore 

 differences which, from their fundamental nature, are not 

 permanent, that is, modifications which are not heritable ; 

 for of such as these evolution is not made. Since inheri- 

 tance depends upon the continuity of material of the 

 chromosomes of the germ cells, changes in adult struc- 

 ture or function can only be permanent when they follow 

 a corresponding change in one or more chromosomes. 

 These chromosomal changes may conceivably arise from 

 within, or be impressed from without. Much of the mod- 

 ern investigation which has a bearing upon the method 

 of evolution is concerned with the question whether the 

 modifications of cliioniosoiiu's are caused by internal or 

 external agciicio. 



Two of the most >nikiiiu' cases of the origin of new 

 heritable cliaracteristic-s arc those of the fruit fly Droso- 

 pliila and the evening primrose CEnothera. Scores of 

 l)eriiianent changes in these organisms have appeared 

 within the last decade. These new features have appeared 

 in one individual among a hundred in the same bottle, or 

 anionu- a thousand in the same tield. Environmental dif- 

 ferenct's >ccui ('\(-Iii(1(m1 in tlioi' cases. For, if one attrib- 

 utes these cliaiiucs to invisible and nnsuspected variations 

 of the environment in <-iivum-ci il)ed regions in a bottle or 

 field, there is no need to appeal, a^ has usually been done, 

 to the grosser element.- like climate and medium, and 



