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? 
8. 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? 
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—1. 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 plastic 
characters in plants and cases of infectious diseases.1 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.” 
10One may refuse to call these cases of inheritance if he chooses to define that term so as to 
exclude them. 
