270 PHYSIOLOGICAL DEVELOPMENT. 



were the phenomena otherwise caused. That the stronger 

 and the feebler contrasts among the different parts of the 

 outer tissues in plants, should so constantly occur along with 

 stronger and feebler contrasts among the incident forces, is 

 in itself weighty evidence that unlike outer actions have 

 caused unlike inner actions, and correspondingly-unlike 

 structures; either by changing the functional equilibrium in 

 the individual, or by changing it in the race, or by both. 



Even in the absence of more direct proof, there would be 

 great significance in the marked differences that habitually 

 exist between the exposed and imbedded parts of plants, be- 

 tween the stems and the leaves, and between the upper and 

 under surfaces of the leaves. The significance of these differ- 

 ences is increased when we discover that they vary in degree 

 as the differences in the conditions vary in degree. Still 

 greater becomes the force of the evidence on finding that 

 these strongly-contrasted parts may, when placed in one 

 another's conditions, and kept in them from generation to 

 generation, permanently assume one another's functions, and, 

 in a great degree, one another's structures. Even more con- 

 clusive yet is the argument rendered, by the discovery that, 

 where these substitutions of function and structure take place, 

 the superinduced modifications differ in different circum- 

 stances ; just as the original modifications do. The fact that 

 a flattened stem simulating a vertically-growing leaf has its 

 two surfaces alike, while when it simulates a horizontally- 

 growing leaf its upper and under surfaces differ, is a fact 

 which, standing alone, might prove little, but proves much 

 when joined with all the other evidence. And its profound 

 meaning becomes the more obvious on discovering that the 

 same thing happens with petioles when they usurp leaf- 

 functions. 



Finally, when we remember how rapidly analogous modi- 

 fications of function and structure arise in the superficial 

 tissues of individual plants, the general inference can scarcely 

 be resisted. When we meet with so striking a case as that 



