q6 FOOT-NOTES TO EVOLUTION. 



And, as Dr. Amos Griswold Warner has wisely observed, 

 no species and "no race ever became extinct through 

 an excess of brotherly love." 



VII. Isolation. — A great factor in the production of 

 variant forms is the isolation of groups of individuals 

 from the mass of their species. The barriers of the 

 earth, separating one group of individuals from other 

 individuals of the same kind, cause them to be exposed 

 to different influences. The reaction from environment 

 is different in one case from another. As a result, the 

 presence of barriers shows itself in specific variation. 



Each species of animal or plant tends to extend and 

 to cover the world. That a given species has not occu- 

 pied any certain area is due to one of three causes: 

 either (a) the species has never entered the district ; or 

 {b), having entered it, it could not maintain itself; or (<:), 

 having maintained itself the changed conditions have 

 made of it another species. 



Thus we may say that the reason why the civet cat 

 is not found in New England is because it has never 

 been able to reach that district in its movements. The 

 skylark, which has been brought there, has not main- 

 tained itself because, in the individual cases at least, it 

 could not; while the European rabbit, introduced years 

 ago into Porto Santo in the Madeiras, does not exist 

 because its descendants are so much altered that we can 

 not recognise them as the same species. 



With one of these three general propositions, self- 

 evident, no doubt, all the facts of geographical distribu- 

 tion may be connected. Each species extends its range 

 wherever it can, maintains itself if it can, and undergoes 

 change wherever its members are brought into new 

 conditions or separated by barriers from the mass of 

 their kind. 



The characters to be attributed directly to isolation 



