DIVIDING FACTORS. ISOLATION 431 



"Isolation" used in the broadest sense. — The term "isolation" is 

 a somewhat unfortunate one to use for the type of factor we are here 

 dealing with. It implies physical separation in the geographical sense, 

 whereas there are very many types of isolation not at all based upon 

 spatial separation. In the broadest sense, any factor may be said to 

 play an isolating role if it interferes with free interbreeding among all 

 the members of a species. Completely free interbreeding within the 

 entire range of a species would result sooner or later in an equal dis- 

 tribution among the individuals of the species of all viable hereditary 

 changes. Unless something were to interfere with free interbreeding 

 within the species, it would remain indefinitely one evolving unit and 

 would not subdivide. 



But there are a great many ways in which free interbreeding may 

 be interfered with. Among the most important isolating agents are 

 the following: (a) geographic isolation, involving the setting-up of 

 geographic barriers between subdivisions of the species; (b) sheer dis- 

 tance apart of extreme sections of a species covering a large territory, 

 not involving any other isolation agencies; (c) climatic differences 

 within the range of the species; (d) physical differences in the environ- 

 ment, such as differences in water, soil, sunlight, elevation of land, 

 etc. ; (e) biotic differences in the environment, involving the presence 

 of various other animals and plants within the range of the species in 

 question; (/) reproductive differences among individuals, due to genet- 

 ic changes, that alter the developmental rhythm or the copulatory 

 apparatus, bringing about assortative mating ; and (g) psychic differ- 

 ences, that result in what is known as clannishness among human 

 beings, according to which changed forms tend to mate with their own 

 kind more readily than with others. 



All these, and doubtless other agencies, promote isolation of small 

 or large groups within the species and tend to split the species up into 

 a number of more or less separately breeding groups. 



Isolation and inbreeding. — One of the inevitable consequences of 

 the isolation of a relatively small group from the main body of the 

 species is inbreeding, more or less close. In extreme cases of geographic 

 isolation a new race may be derived from one pair of individuals. 

 The offspring of such a pair, if they breed at all, must breed together, 

 and for successive generations there will be much close inbreeding. 

 Geneticists have demonstrated that continuous inbreeding results in a 

 decrease of heterozygous and an increase in homozygous individuals. 

 Now, even if no new mutations occurred nor any selection of types due 



