480 



THE AMERICAN NATURALIST [Vol. XL1II 



Beginning with some of the most obvious cases, the 

 winter annuals of southern Arizona present an instance 

 of as unmistakable competition of individuals with indi- 

 viduals and species with species as can be found in the 

 eastern forest region of the United States. As the 

 warmth of spring follows the winter rains the ground is 

 thickly carpeted with Amsinckia, Pectocan/a, Boivlesia, 

 and various other herbaceous plants, which stand thick 

 together and present to the eye the familiar crowding 

 which is seen in a field of grain too thickly sown. Cer- 

 tain individuals dwindle and finally die, robbed of water, 

 food and light by their stronger competitors. It might 

 be interesting to repeat the experiment in the laboratory 

 and to tabulate the results statistically, but it could 

 hardly add to the conclusiveness of the demonstration. 

 The same is true of the manifest competition of species 

 with species, as seen for example in the occupation of 

 relatively extended areas by some of the perennial 

 grasses which, but for their presence, would certainly be 

 covered, as the adjacent areas are, by a thick growth of 

 other plants. Here the actual advance of the grasses 

 from year to year may be observed, and such observa- 

 tions for the sake of more definite statement are now in 

 progress on the Desert Laboratory domain. Convincing 

 evidence of competition is thrust upon one's attention in 

 passing from the desert to areas beyond its borders, and 

 if the transition is abrupt, as for example on the western 

 edge of the Salton Basin, where the desert abuts almost 

 upon a mountain wall, the case is all the more striking. 

 In this instance a straight course of less than five miles 

 brings one from the actual desert, with its characteristic 

 sparse growth of salt bushes, creosote bush, galleta grass, 

 and the like, to the chaparral of the mountainside. Along 

 the way the desert species fall out one by one, and are re- 

 placed by elements of the chaparral. As far as can be 

 judged by their habits elsewhere and from their known 

 range in altitude, there is absolutely no reason for this, 

 except their inability to compete with plants of the cha- 



