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THE AMEBIC AN NATURALIST [Vol. XLIII 



tion according to which the relations of desert plants to 

 each other have been so persistently overlooked or at 

 least underestimated. It may now be set down as an 

 established fact that over a large part of the arid or semi- 

 arid territory of the southwest, competition on the one 

 hand and accommodation on the other have much to do 

 with the association of plant species and the density of 

 the plant cover. Far more, it would seem, than has 

 usually been thought, the character of various associa- 

 tions in this region is determined not simply by the 

 physical but also by the living environment. More than 

 ever too, it is plain that the path of progress lies in the 

 direction of applying to the plant itself, in its natural 

 surroundings, the experimental methods of the physical 

 laboratory. Notable and fruitful beginnings have been 

 made in this direction, but one who has attempted quan- 

 titative work with the sahuaro or ocotillo in the open 

 need not be told that it involves difficulties not presented 

 by seedlings of Vicia faba grown in pots, and that prog- 

 ress will necessarily be slow. 



Thus far adjustment and adaptation have not directly 

 entered into the discussion, although a moment's thought 

 shows that all the paths along which we have come con- 

 verge right here. If one variety of geranium flourishes 

 in the desert air, while another by its side dwindles and 

 dies, we can only say at present that the latter is not 

 "adapted," or is apparently incapable of "adjustment" 

 to the atmospheric conditions in which it has been placed. 

 We find that plants growing in the wash near the Desert 

 Laboratory do not, as a rule, succeed in gaining a foot- 

 hold on the long slope leading to the hill near by; they 

 are not adapted to the soil conditions there existing; but 

 the creosote bush, which makes its home on these slopes, 

 grows— thanks to its capacity of adjustment— even more 

 luxuriantly in the wash than on its own domain. Simi- 

 larly, certain plants of the salt spots grow better beyond 

 than within their limits; they have become adapted to 

 large percentages of alkali salts, but their capacity of 



