SEVENTH ANNUAL REPORT. 179 



— where it is within the possibilities that R. montanus and R. 

 osborni merge, ahhough there is as yet no evidence of this. 



We have some twenty-odd specimens from the Cassiar Moun- 

 tains, and all clearly indicate a species distinct from the southern 

 form. We are not so fortunate in our specimens from the 

 habitat of R. montanus. So far as known, all Alaskan Caribou 

 belong to the Barren Ground group, in spite of the general im- 

 ])ression to the contrary. R. osborni probably crosses the eastern 

 border for a short distance. On the south coast of Alaska R, 

 stonei is an isolated and clearly defined species, and unless speci- 

 mens are discovered on the mainland it will probably be ex- 

 terminated before we know much more about it. R. granti, 

 inhabiting the extreme west of the Alaskan Peninsula, has, thanks 

 to the agency of man, been separated from its nearest relatives, so 

 that we have lost whatever forms there may have existed inter- 

 mediate between it and its close kindred on the Arctic coast. 



This last example is very suggestive of the manner in which 

 species originated. A group of animals spreading over a large and 

 diversified area slowly evolves variations in conformity with local 

 conditions. As long as there is a continuous intermingling of all 

 the members of the original group, the development of distin- 

 guishing characters is held in check. When for any reason this 

 distribution ceases to be continuous, as by a severance of land con- 

 nections, by the disappearance of water or forest in some particu- 

 lar tract, or by persecution by enemies, and the isolation caused 

 thereby is maintained sufficiently long, the group is broken 

 up, interbreeding ceases, and free play is given to tendencies 

 toward divergence. Perhaps another change in local conditions 

 occurs, resulting in the migration of one of the new groups 

 back into the territory of another. If the isolation has con- 

 tinued sufficiently long to do its work, the two forms are dis- 

 tinct, and we have, side by side, two animals recognized as dif- 

 ferent species. Such is the case at the point where the range 

 of the Barren Ground and the Woodland Caribou groups over- 

 lap. Such is the case in the West wdiere the ranges of the black- 

 tail and the white-tail deer overlap. Such is the case in Alaska, 

 where members of the grizzly and brown bear groups range over 

 the same country. 



The case of the Virginia deer {Odocoilcus virginianus) is in 

 point. This deer ranges from Lower Canada and iNlaine, in. 



