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THE AMERICAN NATURALIST [Vol. L 



ered himself in a position to vouch for the uniformity of environ- 

 mental conditions, not only in eight years of experiments which he 

 had personally superintended, but also in nine previous years of 

 experiments of which he had neither control nor information 

 until they were completed and which were made sometimes in 

 50, sometimes in 100, and sometimes in 150 bird flocks. Are 

 there not here some elements of uncertainty which at least con- 

 done the offense even if they do not excuse the question ? 



I am prepared to accept without question Pearl's statement 

 that date of hatching can not possibly have had anything to do 

 with the rise in average flock production which has occurred be- 

 tween 1908 and 1915, notwithstanding his own previous state- 

 ments on the subject and the evidence which Phillips has pro- 

 duced that date of hatching of ducks affects their adult size. I 

 am prepared to accept the view that this rise was due wholly to 

 genetic changes, but I do not believe that Pearl or any one else is 

 in a position to say to what agencies the decline previous to 190$ 



And now, with Pearl, I turn with pleasure to the general 

 problem of selection and note that our differences are here rather 

 verbal than real. They lie in that philosophic pitfall of causa- 



Pearl can not conceive that selection may cause or occasion or 

 lead to genetic change, though he can readily see how popula- 

 tions may change under its influence. Thus selection may in- 

 crease the proportion of high-grade individuals but it can not, 

 on his view, beyond a limited and fixed point, occasion the pro- 

 duction of individuals of increased grade. With these views I 

 squarely take issue, and I shall try to show that his view is a 

 purely a priori view, while mine is based on both observation and 

 experiment. 



Pearl's reasoning throughout rests on the assumption that the 

 potentiality of a germ cell can not change except by a causeless 

 method, "mutation"; that no extraneous influences can change 

 it. Experience teaches directly the contrary, indicating that 

 germ-cells brought together in fertilization mutually influence 

 each other. Let us consider for a moment Pearl's illustration. 

 He supposes an organism to exist, A 38 , which is producing ga- 

 metes of the uniform value, a 38 , and can not understand how such 

 gametes uniting with each other can ever produce individuals of 

 a higher value, say A 30 . No more can I, if we accept his further 



