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THE AMERICAN NATURALIST [Vol. XLIX 



accumulated results of individual effort in response to a 

 changing environment, and also to the direct action of 

 the environment upon the organism. Darwin and Wal- 

 lace taught us that species originated by the natural se- 

 lection of favorable variations, and under the influence 

 of Weismann's doctrine of the non-inheritance of ac- 

 quired characters the theory of natural selection is in 

 danger of becoming crystallized into an inflexible dogma. 

 In recent years de Vries has told us that species arise 

 by sudden mutations, and not by slow successive changes, 

 while one of the most extreme exponents of "Mendel- 

 ism," Professor Lotsy, lately informed us that all species 

 arise by crossing, and seriously suggested that the ver- 

 tebrate type arose by the crossing of two invertebrates ! 



This curious and many-sided divergence of opinion 

 amongst expert biologists is undoubtedly largely due to 

 the introduction of experimental methods into biological 

 science. Such methods have proved very fruitful in 

 results which at first sight seem to be mutually contra- 

 dictory, and each group of workers has built up its own 

 theory mainly on the basis of observations in its own 

 restricted field. 



Professor Bateson has said in his recently published 

 "Problems of Genetics": 



When . . . we contemplate the problem of evolution at large the 

 hope at the present time of constructing even a mental picture of that 

 process grows weak almost to the point of vanishing. We are left 



so easily satisfied. Our satisfaction, as we now see, was chiefly founded 

 on ignorance. 2 



In view of this striking pronouncement on the part 

 of one who has devoted his life with signal success to 

 the experimental investigation of evolutionary problems, 

 the remarks which I propose to lay before you for your 

 consideration to-day may well appear rash and ill-ad- 

 vised. I cannot believe, however, that the position is 

 really quite so black as it is painted. We must perforce 

 admit that the divers theories with regard to the work- 



2" Problems of Genetic," p. 97. 



