PROFESSOR VIRCHOW AND EVOLUTION. 635 
is easily 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 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 1 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 possibly known to a few of my readers, but which 
here 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 branches, and budded into innu- 
merable leaves. What caused them to do so a power 
