GENE MUTATIONS 41 



indeed, doubtless in the large majority of cases the results are 

 the reverse of adaptive. Individuals produced with unadaptive 

 groupings of genes and characters simply fail to develop, or to 

 reproduce, leaving only such nev^ groupings as can live and 

 develop under the conditions. If by chance any of these have a 

 particularly favorable constitution, these may multiply and 

 become numerous ; in this vi^ay an adaptive result may some- 

 times come to light. Selective elimination occurs on a grand 

 scale in these changes; this is clearly observable in the 

 laboratory and garden studies. Such adaptation as may result 

 is the consequence of selective elimination and selective 

 propagation. 



Furthermore, the tw^o types of genetic variation that wc 

 have thus far considered — recombinations of genes, and 

 changes in the structure of the genetic system — do not involve 

 any change in the constitution of the genes themselves. They 

 merely rearrange the genes that already exist. In order that 

 they shall be effective, they require the existence of many 

 different kinds of genes. In some way diversity must arise 

 among the genes themselves, giving rise to new types of genie 

 materials; otherwise there could be no result from recombina- 

 tions and regrouping of the genes. We are therefore put again 

 upon a search for a third type of genetic variation, which shall 

 change the constitution of the genes themselves. 



2. GENE MUTATIONS 



What have been believed to be the required actual changes 

 in the constitution of genes, giving rise to new types of genes 

 that make possible progressive evolution, have been observed 

 on an extensive scale; they constitute what are known as 

 gene mutations.^ 



In theory or concept, a gene mutation is a change in the 

 constitution of a single gene; a rearrangement of its constit- 



