2 14 THE PLANT WORLD 



But it is not so plain that competition is inoperative in the 

 prompt check at the limits of the chaparral, which the 

 desert species receive in their advance towards the mountains, 

 in fact the well known habits of such a true child of the 

 desert as the creosote bush preclude the supposition that it 

 is prevented from advancing up the slopes beyonci its present 

 limits by physical conditions unfavorable to its growth. It 

 is a plant which grows well up to at least 6,000 ft., and 

 thrives in much moister situations than the slopes which it 

 here fails to ascend, so that its failure to find a foot-hold 

 here seems hardly explainable in any other way than that it 

 is quite unable to engage in a successful competition for a 

 place on their own ground with the constituents of the 

 chaparral. 



If the explanation here suggested proves to be correct, 

 we are confronted by a situation which indicates that instru- 

 mentation, however indispensable in the investigation of 

 ecological problems, is quite inadequate by itself, to ensure 

 their successful solution. It seems plain that plants of the 

 mountain sides fail to extend into the desert because the con- 

 ditions there are unfavorable to their growth, and these 

 conditions are such as can be determined readily by thermom- 

 eters, hygrometers, and other instruments; but, on the other 

 hand, some at least of the plants of the desert are not pre- 

 vented from advancing up the mountain side by any such 

 unfavorable physical conditions, in fact both soil and atmos- 

 pheric conditions are highly fa^■orable for their growth. Their 

 failure to occupy the ground is apparently due to the simple 

 fact that it is already held by other plants better adapted to 

 the mountain sides which pre^■ent their getting a foothold 

 here. In this case no apparatus, however elaborate, and no 

 instrumentation, however painstaking, can take the place of 

 well directed observation and experiment. The case is in- 

 structive in its bearing upon the relations of desert plants to 

 other vegetation, and the evidence it furnishes that on 

 the edge of the desert two very different biological groups 



