de Beer • The Darwin-Wallace Centenary 



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this supply in the supposed hereditary 

 effects of use and disuse. 



This rehance on the effects of use 

 and disuse as a source of variation, 

 without any effect on his main argu- 

 ment, is the only part of Danvin's 

 demonstration that has had to be aban- 

 doned, and he would have welcomed 

 the reasons for it. 



The Mendelian theory of the gene 

 was worked out by T. H. Morgan and 

 his colleagues. It has established, as 

 firmlv as Newton's laws of motion or 

 the atomic theory, that hereditary re- 

 semblances are determined by discrete 

 particles, the genes, situated in the 

 chromosomes of the cells, which are 

 transmitted to offspring in accordance 

 with the mechanism of germ-cell for- 

 mation and fertilization, and conform 

 to distributional patterns known as 

 Mendelian inheritance. 



The genes preserve their separate 

 identity; they collaborate in the pro- 

 duction of the characters of the indi- 

 vidual that possesses them, but they 

 never contaminate each other; thev 

 remain constant for long periods, but 

 from time to time they undergo a 

 change, known as mutation, which in- 

 volves a change in the characters which 

 they control; after this they remain 

 constant in their new condition until 

 they mutate again. It has been conclu- 

 sively proved that the theory of the 

 gene applies to all plants and all ani- 

 mals investigated, and that the muta- 

 tion of genes is the only known way in 

 which heritable variation arises. 



The Mendelian geneticists had to 

 learn two lessons. On the one hand 

 they discovered that although individ- 

 ual genes are associated with particu- 

 lar characters, their control of those 

 characters is also affected by all the 

 other genes, which constitute an or- 

 ganized 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 astro- 

 nomically numerous possibilities of per- 

 mutations. The recombinations have 

 been shown to bring about gradual 

 and continuous changes in the char- 

 acters under the major control of indi- 

 vidual genes. 



The second lesson that Mendelian 

 geneticists had to learn was that al- 

 though the effects of the mutations 

 which they first obser\'ed appeared to 

 be clear-cut, they were already the re- 

 sults of past gene complexes. For these 

 mutations have occurred before, and 

 the gene complexes have become ad- 

 justed to them. 



It is therefore clear that mutations 

 and recombinations of genes provide 

 the supply of variation on which selec- 

 tion acts to cause evolution exactly in 

 the way Darwin's theory requires. 



THE SIGNIFICANCE OF PARTICULATE 

 INHERITANCE IN EVOLUTION 



The particulate theory of inheri- 

 tance which Mendelian genetics has 

 established involves a number of con- 

 sequences of fundamental importance 

 for the problem of evolution. In the 

 first place, the substitution of this 

 quantitative and deterministic science 

 for the vague and baseless notion of 

 "blending inheritance" completely dis- 

 poses of the difficulty under which 

 Darwin labored to account for the 

 necessar}' supply of variation on which 

 natural selection could act. The most 

 characteristic feature of the Mendelian 

 gene is that it never blends, but retains 

 its identity and properties intact for 

 long periods of time until it mutates, 

 after which it remains intact in its new 

 condition until it eventually mutates 

 again. This means that the amount of 

 variation, or variance, present in a 

 population resulting from previous 

 mutations, is not only conserved 



