90 GENETIC VARIATIONS 



between them. The different types existing in nature have been 

 bred and intercrossed, to determine the method of inheritance 

 of the differences, and particularly, to discover w^hether these 

 differences behave like gene mutations. Some of the chief 

 results may be summarized as follows: 



1. The differences between slightly diverse forms are in the 

 main heritable, not mere bodily effects of different envi- 

 ronments. 



2. The slight differences between the different types — di- 

 verse varieties, subspecies or the like — are not like those due 

 to single mutations; they affect many diverse characteristics. 



3. When the different types are crossed, the differences do 

 not behave like single gene mutations. They do not give sim- 

 ple Mendelian ratios, like those yielded when a mutant is 

 crossed with the parent from which it is derived. On the 

 contrary, the differences yield "blending inheritance," the 

 offspring being intermediate in most characters between the 

 two parent types. 



"Blending inheritance" is known in many cases to be due 

 to the dependence of the characters so inherited on many 

 different genes, that differ in the two parents. This shows 

 therefore that the related types found in nature differ in many 

 of their genes; not in a single gene, as a mutant differs from 

 its parent form. 



4. Hence if one of the varieties has arisen from another, or 

 if the two have arisen from a common ancestral type, there 

 have been slight genetic changes in many of their genes. 



5. Such diverse types (varieties or subspecies) differing 

 slightly, but in many of the characteristics, commonly occupy 

 different, usually adjacent, geographical areas. Sometimes a 

 series of such varieties occupy a series of different areas. 



6. Often in such cases there is a relation between the envi- 

 ronmental differences and the varietal differences, of the 



