PROFESSOR VIRCHOW AND EVOLUTION 407 



own forces. In obedience to these they would arrange 

 themselves, and finally assume positions of rest, forming 

 a coherent mass. Let us suppose the breeze, which now 

 causes them to quiver, to disturb the assumed equilibrium. 

 As often as disturbed there would be a constant effort on 

 the part of the leaves to re-establish it; and in making 

 this effort the mass of leaves would pass through different 

 shapes and forms. If other leaves, moreover, were at 

 hand endowed with similar forces, the attraction would 

 extend to them a growth of the mass of leaves being the 

 consequence. 



We have strong reason for assuming that the ultimate 

 particles of matter the atoms and molecules of which it 

 is made up are endowed with forces coarsely typified by 

 those here ascribed to the leaves. The phenomena of crys- 

 tallization, lead, of necessity, to this conception of molecu- 

 lar polarity. Under the operation of such forces the mole- 

 cules of a seed, like our fallen leaves in the first instance, 

 take up positions from which they would never move if 

 undisturbed by an external impulse. But solar light and 

 heat, which come to us as waves through space, are the 

 great agents of molecular disturbance. On the inert mole- 

 cules of seed and soil these waves impinge, disturbing the 

 atomic equilibrium, which there is an immediate effort to 

 restore. The effort, incessantly defeated for the waves 

 continue to pour in is incessantly renewed; in the mo- 

 lecular struggle matter is gathered from the soil and from 

 the atmosphere, and built, in obedience to the forces which 

 guide the molecules, into the special form of the tree. In 

 a general way, therefore, the life of the tree might be de- 

 fined as an unceasing effort to restore a disturbed equi- 

 librium. In tlie building of crystals Nature makes her 



