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THE AMERICAN NATURALIST [Vol. LI 



slightest detectable subspeeific modification, thrive under both 

 the cloudy, humid conditions of the northwest coast belt and 

 under the relatively cloudless, arid conditions of Owens Valley. 

 Factors of humidity, of air and soil, cloudiness, and light in- 

 tensity, seem to avail nothing in cheeking their spread. With 

 such a degree of associational specialization as is exhibited by 

 these birds there is little chance of a serious competitive struggle 

 with other vertebrates, and no evidence of such has been ob- 

 served. As far as California is concerned, the meadowlark's 

 range is thus only limited associationally and zonally, that is 

 by the extent of its particular meadow habitat and by dimin- 

 ished summer temperature below some critical point. 



The meadowlark well illustrates some further facts with re- 

 gard to distribution. In California it is unquestionably on the 

 increase as regards total population. This is due chiefly to the 

 great extension of habitable territory resulting from man's oc- 

 cupancy and cultivation of the land, bare plains, brushlands and 

 even woods being replaced by irrigated alfalfa and grain fields. 

 These the meadowlarks find suitable and invade because of their 

 expansive reproductivity, and soon populate to the fullest ex- 

 tent permitted by the minimum annual food supply. In other 

 words, associational barriers have moved, to the advantage of 

 this particular bird, though at the same time to the disadvan- 

 tage of endemic species of different predilections. I should esti- 

 mate that the total meadowlark population in the San Joaquin- 

 Sacramento basin is now fully three times what it was thirty 

 years ago. 



Animal distribution is not fixed. It changes with the shifting 

 of the various sorts of barriers, and doubtless also as a result of 

 a gradual acquisition by the animals themselves of the power to 

 overstep barriers, as by becoming inured to greater or lesser de- 

 gree of temperature. The power of such accommodation, or in- 

 herent plasticity, evidently varies greatly among different ani- 

 mals ; and at best its operation is very slow. Many species have 

 proved stubborn and have been exterminated, as the factor-lines, 

 or barriers, shifted. By the shifting of, say, two critical factor- 

 lines towards one another, the existence of a species may have 

 been cut off as by a pair of shears. 



