course of ontogeny is not a sure indication of the course of 

 phylogeny". Haeckel called those ontogenetic stages 

 which repeated the phylogeny, "palingenetic," and those 

 which did not, "coenogenetic." Conklin says: "ontogeny 

 recapitulates phylogeny not in all its stages and forms, but 

 in its factors and principles." 



Isolation. By some evolution-philosophers isolation is 

 given a prominent place as a factor in the evolution of new 

 forms, but it is clear that it is not so important as selection 

 and variation. Isolation tends to divergence from the 

 parent forms, and, acting for long periods, develops pre- 

 potency or stability of type. 



By isolation, it is clear, free intercrossing of members 

 of a species is prevented. Romanes believed that organic 

 evolution is impossible without isolation, but he undoubt- 

 edly laid too much emphasis on the swamping effect of free 

 intercrossing on germinal variations. 



"Isolation tends to the segregation of species into sub- 

 species, makes it easier for new variations to establish 

 themselves, promotes prepotency and fixes characters." 



Many examples of the truth of the above statement 

 might be cited from every division of the animal world, 

 but it will suffice to instance the part of isolation in the 

 development of races, such as the Nordic, Alpine and Medi- 

 terranean peoples of the world who maintain their char- 

 acteristics of physical type and of character. It is likely, 

 further, that the characteristic Scot, Irishman, and Welsh- 

 man are largely the result of the action of isolation. 



The breeds of cattle have arisen largely through arti- 

 ficial isolation, when close inbreeding was practised. 

 (Consult writings of D. S. Jordan, Romanes, Wagner and 

 Gulick; also Madison Grant's work "The Passing of the 

 Great Race.") 



In any study of Isolation the following types of barriers 

 must be considered: 



(1) Geographical barriers, such as mountain ranges, 

 oceans or seas, deserts, etc. 



(2) Temporal barriers, where members of a species reach 

 sexual maturity at different times of the year 



(3) Habitudinal barriers, where a species splits into two or 

 more castes with different habits of life. 



(4) Physiological barriers, such as variations in form of the 

 reproductive organs, preventing mating 



(5) Psychological barriers, due to profound antipathies. 



