THE AMERICAN NATUBALIST [Vol. LI 



the other. This may conceivably be explained through 

 the principle of invasion and reinvasion. Grinnell,^^ who 

 is doing so much towards a detailed knowledge of the 

 Pacific coast fauna, has considered this principle and also 

 the part played by barriers in the development of geo- 

 graphic subspecies or races, in connection with the discus- 

 sion of many specific cases of distribution in birds and 

 mammals. Walter P. Taylor,^^ in a recent study of the 

 western beavers, concludes in agreement with others, that 

 migration, geographic isolation with adaptation to local 

 ecological niches, and final reinvasion of earlier-occupied 

 localities, will account for the origin and present distribu- 

 tion of geographic subspecies such as we have been con- 

 sidering. This explanation seems as likely as any other 

 at the present time, but it is beyond the purpose of the 

 present paper to discuss these aspects of speciation in 

 birds and mammals. The intention is rather to show that 

 the problems involved are entirely different from those 

 concerned with another tjrpe of variability to be men- 

 tioned in a moment. It may be pointed out, however, that 

 although the theory of reinvasion as developed involves 

 the conception of races isolated in certain geographic 

 areas becoming gradually modified through environ- 

 mental stress and fixed before the reinvasion takes place; 

 that there is at the present time no definite evidence that 

 fixation actually takes place gradually, in this way or in 

 any other way. If intermediates between the various 

 geographic subspecies do not occur, this may be because 

 definite though small steps in variation are taken from 

 one race to the other, which would do away with the 

 necessity for assuming a long period of isolation during 

 which the gradual development and fixation of the race 



isGrinnell, Joseph, 1914, "An Account of the Mammals and Birds of the 

 Lower Colorado Valley, with Especial Reference to the Distributional Prob- 

 lems Presented." Vniv. Calif. Pull. Zool., 12:51-294, 9 figs., pis. 3-13, 

 and other papers. 



16 Taylor, Walter P., 1916, "The Status of the Beavers of Western North 

 America, with a Consideration of the Factors in Their Speciation," Univ. 

 Calif. Publ. Zool, 12: 4.13-495. 



