ISOLATION IN ORGANIC EVOLUTION 73 



hybrids or quasi-hybrids. Dr. ShuU informs us 

 that several such cases occur in the flora of Cali- 

 fornia. A race or species of higher animals may 

 develop dislikes or infertilities with forms other- 

 wise nearly related. Caton tells us that this is 

 true of deer, which will not cross with other spe- 

 cies unless " demoralized," or relieved of race 

 antipathy, by enforced association. 



LAW OF GEOGRAPHICAL DISTRIBUTION 



Free interbreeding tends to unify or obliterate 

 forms which are fertile with each other. Isola- 

 tion in any form tends to check this process, and 

 hence in negative fashion works to create new 

 forms based on distinctions arising through nat- 

 ural variation and retained through heredity. 

 From this fact arises the rule that closely related 

 forms or nascent species do not as a rule inhabit 

 or rather breed in the same area. This proposi- 

 tion has been termed by Dr. J. A. Allen " Jor- 

 dan's Law of Geographical Distribution." 



The law or generalization has been stated as 

 follows : — 



" Given any species (or kind) in any region, the near- 

 est related species (or kind) is not to be found in the 

 same region, nor in a remote region, but in a neigh- 

 boring district separated from the first by a bar- 

 rier of some sort, or at least by a belt of country, 

 the breadth of which gives the effect of a barrier." 



This law holds good as a general rule among 

 animals. The only exceptions yet indicated are 



