PROFESSOR VIRCHOW AND EVOLUTION. 387 



most complicated cases of magne-crystallic action were 

 immediately shown to be simple mechanical consequences 

 of the principle of diamagnetic polarity. These early 

 researches, which occupied in all five years of my life, 

 and throughout which the molecular architecture of 

 crystals was an incessant subject of mental contemplation, 

 gave a tinge and bias to my subsequent scientific 

 thought, and their influence is easily traced in my 

 subsequent enquiries. For example, during nine 

 years of labour 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"^ the 

 mind by exercise, and the illumination of phenomena 

 by knowledge. 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 mechanical laws. In ancient 



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