THE AMERICAN NATURALIST 



[Vol. XLV 



The keystone of the pure line arch is the proposition 

 that selection is ineffective except as a means of sepa- 

 rating already existing genotypes. If this keystone- 

 proposition be not sound the whole structure of the 

 theory crumbles. 



The propositions of the genotype theory are such that 

 scieiitilic proof or disproof is rendered particularly diffi- 

 cult. By theory selection can not effect a change in a 

 pure line; by a slippery process of reasoning in a circle 

 any results attained by selection are at once discredited 

 by the assertion that the original material was impure. 

 If. on the contrary, any selection experiment is ineffec- 

 tual it is by some process of reasoning quite incompre- 

 hensible to some of us, at once chalked up to the credit 

 of the new theory. If heritable differences appear within 

 a pure line known to be so, these results are also dis- 

 credited by the assertion that the observed change is a 

 mutation or has been produced by the action of the en- 

 vironment. Truly the unbiased investigator is between 

 the devil and the deep sea ! 



The actual experimental data upon which the genotype 

 theory rests are as yet few. Johannsen's conclusions 

 for beans depend chiefly upon the offspring of only 

 nineteen seeds, and so far as I am aware no other in- 

 vestigator has confirmed his results on Phaseolus. 

 Hanel had only twenty-six original Hydra, and Pear- 

 son's analysis of his data with more adequate methods 

 than he used, evidences against rather than for the geno- 

 type theory. Jennings gives us the records of only six 

 selection experiments involving altogether only a few 

 actually selected Paramecin. Considering the large en- 

 vironmental and growth factors, his conclusions can not 

 be considered as beyond question. 33 The work of Pearl 

 and Surface with poultry and maize seems to me to have 



