flowers are in large masses, be attracted by them from distances at which the flowers themselves are invisible. And manifestly, the flowers which so attract them from the greatest distances, increasing thereby their chances of efficient fertilization, will be most likely to perpetuate themselves. That is to say, survival of the fittest must tend to produce perfumes that are both more powerful and more attractive. These physiological differentiations, then, which mark off the foliar organs constituting flowers from other foliar organs, are the consequences of indirect equilibration. They are not due to the immediate actions of unlike incident forces on the parts of the individual plant; but they are due to the actions of such unlike incident forces on the aggregate of individuals, generation after generation.[48] § 276. The unity of interpretation which we here find for phenomena of such various orders, could hardly be found 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, between the stems and the leaves, and between the upper and under surfaces of the leaves. The significance of these differences 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 conclusive yet is the argument rendered, by the discovery that, where these substitutions of function and structure take place, the superinduced modifications differ in different circumstances; 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.