342 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
tate again. It has been conclusively proved that the theory of the 
gene applies to all plants and all animals investigated, and that the 
mutation of genes is the only known way in which heritable variation 
arises. 'The modifications resulting from good or bad food supply, or 
from the climatic conditions in which plants and animals live, are not 
inherited and are therefore without significance in evolution. 
The history of the reception of Mendelian genetics after its dis- 
covery has been peculiar. The earliest mutations discovered, often 
called “sports,” were usually deleterious and showed marked and dis- 
continuous steps instead of the gradual and continuous variation 
which Darwinian selectionists looked for as the raw material of 
evolution. Selectionists therefore rejected Mendelian genetics as the 
source of variation. On the other hand, the Mendelian geneticists, 
knowing that their mutations were the only source of heritable varia- 
tion, thought that as they showed wide discontinuous steps and arose 
suddenly, readymade and apparently without long-continued selec- 
tion, selection was inoperative in evolution, and they rejected it. 
With the progress of knowledge it gradually became obvious that 
each of these two schools of research objected to the other for reasons 
which were baseless. As more and more genes were identified and 
their effects studied, it became clear that the wide and discontinuous 
mutations first cbserved were the more easily detected extremes of a 
range in which the majority exert only slight effects. For the same 
reason, these mutations were deleterious because organisms are 
delicately adjusted systems, more likely to be upset by large and 
discontinuous changes than by small and gradual steps. 
The Mendelian geneticists also had to learn two lessons. On the 
one hand they discovered that although individual genes are associated 
with particular characters, their control of those characters is also 
affected by all the other genes, which constitute an organized gene 
complex. As a result of previous mutations, gene complexes of 
plants and animals in nature contain many genes, and these are sorted 
out and recombined at fertilization in astronomically numerous pos- 
sibilities of permutations. These recombinations have been shown to 
bring about gradual and continuous changes in the characters under 
the major control of individual genes. Sir Ronald Fisher demon- 
strated the significance of this by showing that a mutant gene that now 
exhibits the quality known as dominance has gradually become domi- 
nant from a previous intermediate condition. This is what has hap- 
pened to those mutations that confer a benefit on their possessors, and 
in their case there has been a selection of gene complexes in favor of 
those which accentuate the effects of a favorable mutant, so that these 
effects are manifested even if the mutant gene is inherited from only 
one parent, which is the definition of dominance. Conversely, with 
genes that place a handicap on their possessors, there has been a selec- 
