PUOFES80H VIHCUOW AND EVOLUTION. 035 



is easily traced in rny subsequent inquiries. For example, 

 during nine years of labor on the subject of radiation, 

 heat and light were handled throughout by me, not 

 as ends, but as instruments by the aid of which the mind 

 might perchance lay hold upon the ultimate particles of 

 matter. 



Scientific progress depends mainly upon two factors 

 which incessantly interact the strengthening of the mind 

 by exercise, and the illumination of phenomena by knowl- 

 edge. There seems no limit to the insight regarding 

 physical processes which this interaction carries in its train. 

 Through such insight we are enabled to enter and explore 

 that subsensible world into which all natural phenomena 

 strike their roots, and from which they derive nutrition. 

 By it we are enabled to place before the mind's eye atoms 

 and atomic motions which lie far beyond the range of the 

 senses, and to apply to them reasoning as stringent as that 

 applied by the mechanician to the motions and collisions 

 of sensible masses. But once committed to such concep- 

 tions, there is a risk of being irresistibly led beyond the 

 bounds of inorganic nature. Even in those early stages of 

 scientific growth, I found myself more and more compelled 

 to regard not only crystals, but organic structures, the 

 body of man inclusive, as cases of molecular architecture, 

 infinitely more complex, it is true, than those of inorganic 

 nature, but reducible, in the long run, to the same mechan- 

 ical laws. In ancient journals I find recorded ponderings 

 and speculations relating to these subjects, and attempts 

 made, by reference to magnetic and crystalline phenomena, 

 to present some satisfactory image to the mind of the way 

 in which plants and animals are built up. Perhaps I may 

 be excused for noting a sample of these early speculations, 

 already possibl} 7 known to a few of my readers, but which 

 hero finds a more suitable place than that which it formerly 

 occupied. 



Sitting, in the summer of 1855, with my friend Dr. 

 Debus under the shadow of a massive elm on the bank of 

 a river in Normandy, the current of our thoughts and 

 conversation was substantially this: We regarded the tree 

 above us. In opposition to gravity its molecules had 

 ascended, diverged into brunches, and budded into innu- 

 merable leaves. What caused them to do so a power 



