PROFESSOR VIRCHOW AND EVOLUTION 405 



traced in my 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 per- 

 chance 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 phys- 

 ical 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 col- 

 lisions of sensible masses. But once committed to such 

 conceptions, there is a risk of being irresistibly led be- 

 yond 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 struct- 

 ures, 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 mechanical 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. Per- 

 haps I may be excused for noting a sample of these early 

 speculations, already possibly known to a few of my read- 



