AN ANALYSIS OF THE EFFECT OF SELECTION. 37 



Mendelian differences are still arising by mutation and may arise 

 in a selection experiment as well as anywhere else; and those that arise 

 in such an experiment are as likely to affect the character under ob- 

 servation as are any Mendelian differences taken at random. It is 

 therefore probable that selection sometimes makes use of variations 

 that arise during the course of the experiment, or, rather, that varia- 

 tions which may be available do arise. 



The question is, wha.t is the relative frequency of the two kinds of 

 available factor differences those already present and those that arise 

 de novof The answer is found by investigation of the data on selection 

 in inbred lines and in crossbred lines. In closely inbred strains there are 

 not likely to be many factor differences present when selection is begun, 

 while in crossbred lines these differences are likely to be numerous. 



That selection is usually effective in crossbred lines is a well-known 

 fact, demonstrated many times with many different organisms. Not 

 many experiments have been carried out on closely inbred material, 

 but those of Johannsen (1903), MacDowell (1917), and the present 

 paper (p. 11) show that selection may be without effect in such lines. 

 In two of these cases selection was effective until the lines became highly 

 inbred. But mutations influencing the characters under observation 

 have been obtained in the selection experiments of Castle and Phillips 

 (1914), Morgan (Morgan, Sturtevant, Muller, and Bridges, 1915, 

 p. 205), Lutz (1911), and those reported in this paper (p. 31). * 



Apparently, then, selection produces its effects chiefly through 

 isolation of factors already present, but occasionally available muta- 

 tions do arise during the course of the experiment. 



2. Does selection cause mutations, or influence their direction? 



The usual selection experiment consists in breeding from individuals 

 that are extreme in some respect. This extreme character may be 

 environmental in origin, or it may be caused by germinal differences. 

 In the first case, no geneticist is likely seriously to maintain that selec- 

 tion will have any effect whatever. In case the extreme character 

 is germinal in origin, selection will of course be effective in eliminating 

 certain genetic types. Moreover, given a combination of genes that 

 produce the character in a certain degree, we are evidently in a better 

 position to reach a further stage than if we have the character less well 

 developed. For how long a tail will be when it gains an inch evidently 

 depends on how long it was before it gained that inch. But it seems 

 incomprehensible that selection of individuals of a constitution favor- 



1 Evidence derived from forms that reproduce asexually is also available in studying this 

 question, for such reproduction commonly prevents recombination, and therefore gives results 

 comparable with those obtained from homozygous strains. Some of the evidence obtained from 

 studies on asexually produced Protozoa (e. g.. Calkins and Gregory, 1913; Jennings, 1916; Middle- 

 ton, 1915) has shown that selection may be very successful in changing such forms. But it is 

 very doubtful if these animals are comparable with the Metazoa in the method of distribution 

 of their chromatin. It seems not improbable that in some cases recombination may here be 

 possible in asexual reproduction. 



