CHAPTER XXII 



ISOLATION— SPATIAL AND PHYSIOLOGICAL AND DETERMINATE 



EVOLUTION 



Isolation and natural selection in evolution. Natural selection and the 

 origin of species. Isolation and geographical distribution. Variations blastogenic. 

 Romanes and Isolation. Physiological selection. Inter-breeding species. Island 

 forms. Species formation in high altitudes. Discriminate isolation. De- 

 terminate evolution. 



THAT organisms possess a tendency to vary in all direc- 

 tions is now generally admitted as beyond dispute, 

 nor is it denied that the only limitation to these 

 variations, the only check upon their undue development, 

 whereby the balance of the whole organism would be endangered, 

 is that imposed by natural selection. 



Natural selection, in other words, is charged rather with the 

 execution of Nature's sumptuary laws — with the control or 

 limitation of these variations — than, it would seem, with their 

 origin and causation. And while some of these variations 

 affect the internal policy of the organism, and are often, and 

 indeed generally, too subtle to be readily discerned, others 

 determine its relations to the outer world ; and among these 

 last are to be reckoned the "specific characters" of an animal, 

 the sum of the characters by which one group of individuals 

 possessing such and such characters in common may be dis- 

 tinguished from another group presenting similar but different 

 characters. 



Stable as these specific characters appear to be, under 

 certain conditions, they can be shown to undergo more or less 

 marked transmutations and thereby a species A may " bud off" 

 so to speak a species B. This process of evolution can be 

 shown to exist whether we follow the history of any given 

 organ through a large number of different and unrelated species 

 treating the organ as if it were an individual, or whether we 



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