CHAPTER XXXIII 



DIVIDING FACTORS. ISOLATION 



Introductory. — As has so often been pointed out, one of the most 

 conspicuous features of animal and plant life is their subdivision into 

 a multiplicity of taxonomic divisions, such as phyla, classes, orders, 

 families, genera, species, and varieties. In any one particular environ- 

 ment one finds large numbers of representatives of numerous groups, 

 a fact that seems to imply that the environment alone is an insufficient 

 cause of multiplicity of forms, for if it were, a single environment 

 should produce only one type, and the same environment should always 

 produce the same type. Also, it is very frequently true that differ- 

 ences distinguishing two allied species are not obviously adapted to 

 the differences in the environment, but are rather trivial characters 

 such as color markings, proportions of parts, etc. Hence we cannot 

 explain the multiplicity of types on purely environmental grounds. 



Our studies of geographic distribution as evidences of evolution 

 have emphasized the fact that mere geographic isolation, apart from 

 climatic differences involved, are invariably accompanied by various 

 degrees of divergence between the isolated members of the same group, 

 the divergence sometimes being so great as to constitute family dis- 

 tinctions, more frequently generic, and very commonly specific or 

 varietal. The degree of divergence parallels closely the degree of 

 completeness of isolation and the extent of time during which isolation 

 has been operative. 



Opinions among authorities differ as to whether mutations and 

 selection alone are sufficient to account for the multiplicity of forms 

 and their distribution in space. Some of the more extreme neo- 

 Darwinians, on the one hand, are inclined to believe that natural selec- 

 tion is sufficient unto itself to explain all of the observed facts. Extreme 

 advocates of the isolation theory, on the other hand, look upon isola- 

 tion as an absolutely essential mechanism for species formation nearly 

 as important as natural selection itself. The writer believes that iso- 

 lation is an absolutely essential part of any complete explanation of 

 evolution, but that it is a subsidiary factor often working in such inti- 

 mate relation with natural selection and Mendelian heredity as to be 

 almost inseparable from them. 



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