96 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. Lsolation—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. Asa 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 (2) the species has never entered the district; or 
(4), 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 
