DARWIN-WALLACE CENTENARY—prz BEER 343 
tion of gene complexes in favor of those which suppress the effects of 
such genes so that they are manifested only when the mutant gene is 
inherited from both parents, which is the definition of recessiveness. 
They may be suppressed even further, as when the effects of such a 
gene are obliterated and the gene becomes what is known as a “modi- 
fier,” without major control over characters. It has even been 
demonstrated by E. B. Ford, under rigorous experimental conditions, 
that one and the same mutant gene can be made to become dominant 
in one strain and recessive in another, simply by selecting as parents 
those individuals whose gene complexes accentuate or diminish the 
effects of the gene. 
The second lesson that Mendelian geneticists had to learn was that 
although the effects of the mutations which they first observed ap- 
peared to be clear-cut, they were already the results of past gene 
complexes. For these mutations have occurred before, and the gene 
complexes have become adjusted to them. The fact that a single gene 
may now act as a switch controlling the production of one or another 
character difference does not mean that this character difference orig- 
inally arose at one stroke by one mutation of such a switch gene, be- 
cause it has probably been built up gradually as a result of past selec- 
tion in the gene complex. 
It is therefore clear that mutations and recombinations of genes 
provide the supply of variation on which selection acts to cause 
evolution exactly in the way Darwin’s theory requires. Its require- 
ments are exacting, for, as T. H. Huxley pointed out, some organisms 
have evolved slowly and others have evolved fast; he saw that natural 
selection was the only mechanism that could satisfy both those re- 
quirements. It is able to do so because Mendelian inheritance is 
capable of producing both diversity and stability. As Ford has 
said, an immense range of types must be available for natural selection 
to act upon, and this is provided by mutation and recombination of 
genes. Yet when a favorable gene complex has been achieved it 
must not be dissipated and broken down, and this is provided against 
by the facts that the genes do not blend or contaminate one another, 
and that they mutate only rarely. 
THE SIGNIFICANCE OF PARTICULATE INHERITANCE IN EVOLUTION 
The particulate theory of inheritance which Mendelian genetics 
has established involves a number of consequences of fundamental 
importance for the problem of evolution. In the first place, the sub- 
stitution of this quantitative and deterministic science for the vague 
and baseless notion of “blending inheritance” completely disposes of 
the difficulty under which Darwin labored to account for the neces- 
sary supply of variation on which natural selection could act. The 
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