Isolation 361 



ing distinct types in a segregated area. Regional varieties 

 or species of birds, mammals and other classes of animals are 

 well known to taxonomists (Fig. 41). David Starr Jordan, 

 who had himself recorded observations on regional types, es- 

 pecially among fish, constantly stressed the importance of 

 barriers to the swamping or merging of variants. He ob- 

 served moreover the tendency of variation to produce dis- 

 tinctive lines under isolation: " As dialects form in human 

 speech, where men cease to mingle generally, so species form 

 among animals or plants where there exists a check to 

 migration." 



Another suggestion to meet this difficulty is the theory 

 of physiological isolation. If two or a few individuals show 

 a distinct trait this particular quality can be preserved in 

 future generations only if those having the character mate 

 together and avoid mating with the mass of individuals not 

 showing the trait. We have since found, however, that if 

 a particular character appears in a heritable form in a single 

 individual, it would be possible for it to reappear in subse- 

 quent generations, and eventually to become established, 

 even though the mate of the first variant did not show the 

 same trait. A strain of wheat was discovered in England 

 to be immune to rust. This quality was found to be a Men- 

 delian recessive. By crossing this immune wheat with other 

 varieties, it is possible to combine the quality in question with 

 other desirable traits in a new strain. 



Isolation as a factor in evolution may be considered 

 important regardless of whether natural selection itself 

 proves to be as effective as Darwin considered it. In many 

 observations on the development of human qualities in races, 

 tribes, or sects, isolation plays an important role. Eugenists, 

 who are for the most part disposed to follow the Darwinian 

 interpretation of the evolutionary process, stress the persist- 

 ence of distinct groups of traits in segregated human stocks, 

 such as the famous Jukes family, the Parsees of India, the 

 Icelanders. They fail, however, as a rule, to distinguish 

 clearly between the hereditary factors that repeat them- 



