DARWIN-WALLACE CENTENARY—DE BEER 341 
THE INTEGRATION OF MENDELIAN GENETICS WITH SELECTION 
When Darwin wrote, nothing whatever was known about the laws 
of heredity, and all that he had to go on was the vague notion that 
offspring tended to strike an average between the characters of their 
parents. This supposition went by the name of “blending inherit- 
ance,” and it occasioned for Darwin the greatest difficulty with which 
he had to contend in formulating his theory. In the first place, if 
blending inheritance were true, it would mean that any new variation 
which appeared, even if heritable, would be rapidly diluted by 
“swamping,” and in about 10 generations would have been obliterated. 
To compensate for this it would be necessary to suppose that new 
variations were extremely frequent. Since whole brothers, sons of 
the same father and mother, share an identical heredity, any differ- 
ence between them would have to be due to new variation that had 
arisen during their own early lives, and variation would have to 
affect practically all members of a species. This problem of the 
supply of variation was a difficulty which Darwin felt so acutely 
that it even led him to look for a source of this supply in the supposed 
hereditary effects of use and disuse. 
This reliance on the effects of use and disuse as a source of varia- 
tion, without any effect on his main argument, is the only part of 
Darwin’s demonstration that has had to be abandoned, and he would 
have welcomed the reasons for it. If only Darwin had realized it, 
the solution to all these difficulties was at that very time being provided 
by Gregor Mendel, but his results were unknown until 1900, eighteen 
years after Darwin’s death. 
The Mendelian theory of the gene was worked out by T. H. Morgan 
and his colleagues with an unprecedented wealth of experimental 
evidence from the breeding pen and from cytological studies on the 
structure of the cell and its chromosomes. It has established, as 
firmly as Newton’s laws of motion or the atomic theory, that hereditary 
resemblances are determined by discrete particles, the genes, situ- 
ated in the chromosomes of the cells, which are transmitted to offspring 
in accordance with the mechanism of germ-cell formation and fer- 
tilization, and conform to distributional patterns known as Mendelian 
inheritance. The researches of C. D. Darlington and others on the 
structure and behavior of the chromosomes have reached such a degree 
of refinement and precision that each step in the mechanism of 
Mendelian inheritance can actually be seen under the microscope. 
The genes preserve their separate identity; they collaborate in the 
production of the characters of the individual that possesses them, 
but they never contaminate each other; they remain constant for long 
periods, but from time to time they undergo a change, known as muta- 
tion, which involves a change in the characters which they control; 
after this they remain constant in their new condition until they mu- 
