96 



SCIENCE. 



[N. S. Vol. XXI. No. 525. 



conforms exactly to the surface, to soil and 

 level, no doubt with an exactness that we 

 have only begun to guess or understand. 

 There is a mathematical line that limits the 

 distribution of every plant, but the area 

 forever shifts and varies. The topog- 

 raphy varies, except the 7nal pais, by 

 changes so slight, so delicate, as to be im- 

 perceptible to eyes unskilled, and with the 

 topography varies its covering of life. Let 

 as say first that these topographic changes 

 will change the limits of distribution. Once 

 the sands cover the silt plains, and the 

 grasses will vanish while yucca and arte- 

 niisia succeed. AViden the talus and 

 eovillea will stretch farther its golden 

 scepter. But the problem runs far deeper 

 than this. As the face of the world under- 

 goes these delicate, subtle changes, the 

 plant responds in something far more than 

 shifting distribution. A plant, as every 

 student of botany well knows, is the most 

 plastic sort of an organism in the Avorld, 

 responding in every sort of way to its 

 environment. We who study the micro- 

 scopic structure of the humblest plants 

 understand the limitless possibilities here. 

 When we retlect that the suppression of a 

 single cell at the critical moment may 

 change the direction of the axis or alter the 

 contour of a leaf, it is hard to set too high 

 an estimate upon the possible response 

 made by a simple plant to environmental 

 variations, however delicate. We who 

 study the physiology of the plant, peer into 

 its changing cells and strive in imagination 

 to reproduce the marvelously intricate re- 

 actions, physical, chemical, that forever 

 shift and play within those narrow limits— 

 we need not be told that every vegetable 

 cell has in it opportunities a thousandfold 

 to match and meet all the subtle changes 

 suggested by the slow-creeping but implac- 

 able forces that work out the physiognomy 

 of this time-worn earth. A little more cal- 

 cium here, a little more phosphorus there, 



sulphates, nitrates and the rest, and the 

 thing is done. Nay, when we even think 

 of the form in which all energy comes 

 from yon distant sun, and the delicate 

 machinery on which it plays, we need seek 

 no further occasion for the intervention of 

 every sort of outer cosmic force. Not a 

 tree on all the Iowa prairies but shows in 

 its every lineament, in its very expression, 

 a response to the Iowa environment ; and so, 

 we may be sure, every desert plant records 

 in its present form and stature all the af- 

 firmations, all the responses it has made in 

 all the centuries to the bidding, the silent 

 bidding, the most gentle coaxing, of the 

 world external. For, note you, the call for 

 change at any given instant has not been 

 great ; the slow upheaval of these moun- 

 tains, their peaceful, gentle removal by the 

 winds and rain; that is all; but that has 

 changed and is changing the living world. 

 Where the terrestrial call is rude or sud- 

 den, response there is none. The lava beds 

 show no single characteristic species. Their 

 flora is simply that of their own rocky 

 level. Nor could here any sudden initia- 

 tive on the part of the plant avail. The 

 adaptation is absolute now, and to vary 

 save as the environment varies would sim- 

 ply invite disaster. As well the tadpole 

 suddenly assume lungs or the lizard put 

 on feathers. 



Nor is this all— our desert as it lies 

 shining here before us is but a fraction of 

 that wider, vaster desert that covers all the 

 south and west. Across the Organ and 

 San Andreas yonder is another desert ex- 

 actly comparable to that we .study; all 

 Arizona, southern California, Sonora, Chi- 

 huahua, much the same ; here and there a 

 mountain summit tiifted with forest, west- 

 ern in type, high slopes thinly clad with 

 stunted juniper, benches of eovillea, wide 

 low plains covered with mesquite, with 

 yucca and cactus and all the less noble 

 plants that stand between ; and our prob- 



