64 FREAKS OF PLANT LIFE. 



results in change of form, and usually declares itself 

 in the doing of mechanical work. This property of 

 excitability, which, let me repeat, is common to all 

 living structures, is, as we have seen, comparable in 

 its simplest manifestations to that possessed by many 

 chemical compounds (of explosiveness) and many 

 mechanical contrivances (of going off or discharging 

 when meddled with, as in the case of the rat-trap 

 already referred to). 



" In physiology, as in the other sciences of observa- 

 tion, the process of investigation is throughout one of 

 comparison. Not only do we proceed, from first to 

 last, from the kno\\-n towards the unknown, but what 

 we speak of as our knowledge, or understanding, of 

 any new fact consists simply in our being able to 

 bring it into relation with other facts previously well 

 ascertained and familiar, just as the geographer deter- 

 mines the position of a new locality by ascertaining 

 its topographical relation to others already on the 

 chart. 



" The comparison we have now to make is between 

 the contractility displayed by the leaf of Dionoea, and 

 the contractility of muscle. I choose muscle as the 

 standard of comparison, because it is best known, and 

 has been investigated by the best physicists of our 

 time, and because its properties are easily illustrated 

 and understood. I shall be able to show that the 

 resemblance between the contraction of muscle and 



