LIFE AND ITS PHENOMENA 63 



more wood-fibres and other mechanical tissues are 

 developed to meet the strain ; just as bones and muscles 

 are formed wherever they may be required in animals. 

 A large series of experiments with plants has lately been 

 made by M. Ph. Eberhardt/ to show the adaptations in 

 the minute microscopical characters of the tissues of 

 plants subjected to drought and moisture. In every 

 case the artificially induced results were precisely the 

 same as observable in plants growing in arid and desert 

 regions on the one hand and in marshes and in water on 

 the other. In all cases the changes are just those most 

 suitable to the conditions under which the plants grow. 



Can, therefore, alterations of structure in response to 

 external forces and in adaptation to them be accounted 

 for solely by means of the physico-chemical processes 

 involved in their manufacture ? There is no such Direc- 

 tivity ever observable, I repeat, in the mineral kingdom. 

 The nearest approach to it is seen in the angular 

 symmetry of crystals ; but this is constant. 



Why are the molecules of living bodies being con- 

 stantly arranged differently ? No two peas in a pod are 

 precisely alike, showing a variability which becomes very 

 pronounced under cultivation. 



On the other hand molecules of all inorganic sub- 

 stances assume constant forms when crystallised. A 

 crystal of salt made artificially is exactly like their 

 " casts " found in the Triassic strata. Every substance 

 that can be crystallised can be referred to its "system," 

 whether it be quartz from the silurian rocks or of any 

 later strata. 



Evolution, on the other hand, shows constant change 

 in living organisms. Whence is this difference .? 



^ Ann. des Sci. Nat., xviii,, p. 61, 1903. 



