No. 606] 



METHOD OF GENETICIST 



367 



is plainly tlie case in Drosopliila to-tlay, there is no reason 

 to deny that the invisible modifications of chromosomes, 

 if such occur, are likewise of internal origin. 



In suggesting possible sources of internal cliaiige r<'sult-. 

 ing in evolution I am not blind to the fact that an iiltiiiiate 

 explanation of the method of evolution i> not tlicirhy 

 offered. The chemical processes whicli cause these ])]ie- 

 nomena, while they are distinctly within the field of the 

 geneticist, are not within his knowledge. If there were 

 any prospect that an ultimate solution of the problem of 

 the causes of germinal clianges could be offered at the 

 present time, invitation to ] lartici] tatc in tliis discussion 

 should have been extended to a j »hy>ioh)i:i>t ; for it is 

 from him that the eventual explanation of these internal 

 changes must come. 



In this account of possible ways in which changes in 

 the chromosomes of germ cells arise, I have not forgotten 

 that it is conceivable that the changes are forced by 

 external agents. There ai e, indeed, biologists who regu- 

 larly attribute >ueh ehanues to environment. The paleon- 

 tologists not inl're(iuently seem to regard evolution as 

 ordinarily so caused. But with a few exeejitions. those 

 who hold these views are not exj •ei iuientali>t>. They are 

 not the biologists who are engagcvl in >tud\ ing ^iresent 

 phenomena. They reason from ie>ults to cause. Out 

 of the conceivable causes the> have pieked on one which 

 has a chance of being the right one. but only a chance. 

 I venture to suggest tlnit the theory of internal origin of 

 modifications will account for all paleontological, morpho- 

 logical, and geographical phenomena, and accord with all 

 evidence from those fields, quite as well as the environ- 

 mental theory. 



Among the experimentalists, it is to be admitted, there 

 are a few who occasionally pnxdaim the di>eovery of a 

 modification produced hy the fn\- iron meat and subse- 

 quently inherited. P>y one or two. not po>>essed of the 

 still small voice, these procdamal ions are made repeat- 

 edly. Sometimes the effect of the environment is admit- 



