GENETICS IN RELATION TO EVOLUTION 485 



tion with certain other genes. The evidence for interaction of 

 genes is clear-cut but very comphcated, and will not be given here. 

 Morgan states that at least twenty-five allelomorphs are con- 

 cerned with eye-color. Conversely, a single pair of genes may 

 influence more than one character. In Drosophila the genes for 

 rudimentary wings affect characters of the legs and number of eggs 

 laid, among others. 



Genes are the hypothetical units that determine inheritance in 

 organisms. For each character of the individual, at least two 

 allelomorphic genes are responsible. There may be many allelo- 

 morphic pairs of genes that interact to produce a given character; 

 a single pair of genes may also influence the appearance of more 

 than one character. The characters, thus determined by genes, 

 require, for their realization in the adult, certain conditions of the 

 environment in which development occurs. The tentative hypoth- 

 esis concerning the nature of genes proposes that they are chemical 

 compounds, and experiments arc being made to test this idea. 

 More definite knowledge awaits additional information concerning 

 the physico-chemical nature of the protoplasm, of which the genes 

 must be a part. We return, therefore, to the generalization that 

 the individual inherits a "course of development." The researches 

 of geneticists have clarified the problems to the extent indicated in 

 the previous detailed discussion. 



Genetics in Relation to Evolution 



Organic Evolution or the origin of present-day organisms from 

 organisms of the past by a series of gradual changes, is dependent 

 upon the inheritance of variations. Offspring must be different 

 from their parents, and the differences must be hereditary, in 

 order that evolution may occur. The Mendelian principles would 

 seem to deal with units that are fixed and unchanged from genera- 

 tion to generation. Yet the variations that Gal ton studied in 

 populations, and Johannsen in pure lines, can be explained in 

 Mendelian terms. 



The variety of recombinations possible in a cross involving three ' 

 pairs of genes gives some indication of the range of genetic varia- 

 tion that occurs when heterozygous individuals interbreed. When 

 one considers that many pairs of genes are necessary to produce 

 certain characters, and that a single pair of genes may affect more 



