PHILOSOPHICAL SOCIETY OF WASHINGTON. 71 



The processes of growth aucl metabolism exhibit different degrees 

 of intensity in accordance with variations of the environment, and 

 whatever physical theory of the mode in which the protoplasmic 

 motions are produced we may adopt, the mechanical force manifested 

 can only be supposed to proceed from the decomposition of a part of 

 the protoplasm itself into simpler compounds, that is, from a particu- 

 lar kind of metabolism. Hence you will I think, be quite prepared 

 to hear me speak of all the circumstances in the environment that 

 so act upon living protoplasm as to increase its growth or meta- 

 bolism, as stimuli, and of the property of living protoplasm by 

 which all its responses to stimuli are guided, as irritability, instead 

 of limiting these terms to the phenomena of automatic movement 

 only, as was formerly done. This irritability of living protoplasm 

 determines the direction in which its internal forces shall be mani- 

 fested. Speaking of it as I do, perhaps you would wish me to call 

 it sensibility rather than irritability, and I do not know that I 

 should object very strenuously to any one who wished to do this. 

 But however you may name it, it is this vital property of all living 

 protoplasm that produces the sensibility to changes in the environ- 

 ment which has been the main factor in the gradual evolution, 

 during the ages, of the highest and most complex from the simplest 

 and lowest living forms. 



Against this view it has been urged with much ingenuity that 

 protoplasm is the material substratum of life, and life merely a 

 property of protoplasm ; that is, if the words have any meaning at 

 all, that life is the resultant only of the forces inherent in the in- 

 organic atoms of which the protoplasm is built up. Now, in the 

 first place, no one has ever yet been able to show, by any conceiv- 

 able synthesis, how the forces known to belong to the several kinds 

 of inorganic atoms of which protoplasm is composed, could by their 

 combination, produce the characteristic phenomena of living pro- 

 toplasm, namely, the phenomena of irritability, as I have just 

 described them. But, in the second place, this speculation appears 

 to be pretty flatly contradicted by the circumstance that, although 

 protoplasm can only be formed within the substance of previously 

 existing living protoplasm, it can continue to exist, it does continue 

 to exist a,s protoplasm after it has ceased to live. Not merely can 

 it persist for a time without chemical change as dead protoplasm, 

 it can subsequently serve as food and be reconverted into living 

 protoplasm once more.' Bear in mind, however, that this change 



