316 THE CONTINUITY OF THE RACE 



tion. 1 Most genes are remarkably stable, existing unchanged for hundreds 

 or thousands of generations. When an occasional individual gene does 

 change, the new mutant appears to have a stability very like that of the 

 original gene from which it came. It is evident that mutations are real 

 occurrences, that they arise spontaneously in the sense that they have 

 no known or predictable cause, and that all of the known allelomorphic 

 sets of genes must have arisen through mutation at some time in the 

 history of the race. 



Multiple allelomorphs. So far, we have been concerned with allelo- 

 morphic pairs of genes; but a rather large number of cases are known in 

 which three or more different genes show an allelic relationship. Dro- 

 sophila again furnishes a good example. Some time after the appearance 

 of the mutation "white-eye," another mutation took place in another 

 individual (and in another generation) of homozygous red-eyed flies that 

 resulted in a "peach-" (light yellowish red) eyed individual. The gene 

 for peach proved to be dominant to the gene for white and recessive to 

 the gene for red, and breeding experiments have shown that all these 

 genes occupy the same locus in the X chromosome. All are sex-linked; 

 any given male Drosophila can have only one of these genes, and any 

 given female can have but two. Other sets of multiple alleles are known in 

 Drosophila, and in rabbits, mice, guinea pigs, man (blood types), and a 

 number of plants. 



Hybridization and new combinations of genes. New inherited varia- 

 tions may also be produced by bringing together new combinations 

 of genes. We have seen that many characters are due to various types 

 of interaction between non-allelic genes, and when certain genes are 

 brought together for the first time, a new character may result without 

 involving any recent mutation. A similar phenomenon is shown in what 

 is known as reversion. Not infrequently, when two domesticated races, 

 derived from the same wild stock, are bred together, the progeny (or 

 some of them) show a "throwback" to the ancestral condition, evidently 

 produced by the reuniting of certain genes that cooperate to produce the 

 original wild type of organism. 



HEREDITY AND ENVIRONMENT 



We have distinguished among variations that are due to environmental 

 causes and those that owe their existence and transmission to genes within 

 the cells. This distinction, although thoroughly justified for analysis and 

 study, gives an incomplete picture of the complexity of the whole prob- 



1 There is much inferential evidence that genes are some sort of protein molecules, 

 or groups of such molecules, and that a mutation must be due to some change in the 

 structure and hence in the reactions of the molecule or molecules that constitute the 

 gene. 



