36 AN ANALYSIS OF THE EFFECT OF SELECTION. 



GENERAL DISCUSSION. 

 THE SELECTION PROBLEM : QUESTIONS AT ISSUE. 



It appears to the writer that the three questions below are the chief 

 ones at issue in the discussion of the selection problem: 



1. Does selection use germinal differences already present, or differences 



that arise during the experiment, or both? 



2. In case it uses new differences, does it cause them to occur more 



frequently, and does it influence their direction? 



3. Are differences, already present or arising de novo, more likely to 



occur in the locus of the gene under observation, or in other loci? 



It is not, I think, questioned by any one that selection may effect 

 either gradual or sudden change in the mean character of mixed races, 

 or that it may even, occasionally, produce such an effect in pure races 

 if a mutation in the desired direction happens to occur. 



1. Does selection use germinal differences that are already present, or differences 

 that arise during the experiment f 



Everyone who has bred animals or plants is familiar with the fact 

 that different strains, even when rather closely related, differ in all 

 sorts of minor points— size, proportions of organs, shade of color, resist- 

 ance to disease, fertility, temperament, rate and habit of growth — 

 in fact, in almost any respect that one investigates. This can only 

 mean that such strains differ genetically; and since the kinds of differ- 

 ences are usually so numerous, they probably usually have many 

 genetic differences — i. e., they differ in respect to many factors. In 

 any race not normally self-fertilizing or closely inbred, crosses between 

 individuals of different constitution must then be frequent. And 

 such crosses must, on the assumption that the original differences were 

 Mendelian, lead to the production of a population more or less hetero- 

 zygous for factors that produce minor effects on all sorts of charac- 

 ters. The assumption that the differences are Mendelian rests on the 

 observed facts, (1) that demonstrably Mendelian factors may produce 

 effects on practically any kind of character studied, and effects of 

 practically any observable degree ; and (2) that non-Mendelian inher- 

 itance has never been demonstrated, except for a few cases of plastid 

 characters in plants and cases of infectious diseases.^ Other kinds 

 of inheritance may exist ; but the available data indicate that they must 

 be extremely rare. Therefore the chances are that any observed 

 difference between two strains is Mendelian. 



If these conclusions be accepted, it follows that any strain not very 

 closely inbred is likely to be heterozygous for factors influencing many 

 characters. Selection for these characters will then be effective in 

 isolating favorable combinations of such ''modifying factors." 



^One may refuse to call these cases of inheritance if he chooses to define that term so as to 

 exclude them. 



