354 



THE AMERICAN NATURALIST [Vol. XLY 



parent and offspring in a population and selects to in- 

 crease or decrease it. He will get no result of selection. 

 If now he takes the same character and selects from the 

 plus and minus variations within a pure line, he will 

 again effect no change by selection. Does either of these 

 cases prove that selection in general is ineffective! Or 

 does the second support in any way Johannsen's geno- 

 type theory of heredity? Certainly not. 



Certain important work of Pearl and Surface seems to 

 me to deserve mention in this connection. 19 These re- 

 searches are sometimes referred to as furnishing evi- 

 dence against the possibility of improvement by selec- 

 tion, and this they do so far as the character with which 

 they have dealt is concerned. In the generalization of 

 their results, however, the greatest caution must be used. 



From two series of experiments with the same strain 

 of Barred Plymouth Rock fowls they show that there is 

 little hope of increasing the egg-laying capacity by direct 

 selection for fecundity. These results are doubtless of 

 much practical importance. Biologically they are of in- 

 terest in confirming the results of other biometric 

 studies which have shown that for man, horse, swine and 

 mice fertility is very slightly inherited in the population. 

 To consider them as indicating that selection in general 

 is ineffective would be a very grave error, for fertility — 

 so far as we may judge from the statistics so far pub- 

 lished — seems to be a character sui generis in respect to 

 inheritance. To cite these results in support of Johann- 

 sen's genotype theory of heredity, as has sometimes 

 been done, is absurd. 



Is it not possible that Johannsen's results with beans 

 may be due to seed weight being a character which is not 

 inheritable at all in the population, and which can not, 

 therefore, reasonably be expected to be inherited within 

 the pure line? 



"Pearl, R., and F. M. Surface, "Inheritance of Fecundity," Bull. Me. 

 Ag. Exp. Sta., 166, 1909. Pearl, R., and F. M. Surface, "Is there a Cumu- 



257-275, 1909. 



