I0 INTRODUCTION 



to it, just as a fish, adapted only to a life in water, is unable to live in air. 

 Whatever may have been the primary causes of the varied character and 

 dissimilar inherent characteristics of different plants, throughout their 

 genealogical history in successive geological periods, we may feel assured 

 that only such modifications as have been of service to the plant have 

 been retained, and that all purposeless or disadvantageous ones have been 

 gradually eliminated. This simple but comprehensive principle was ex- 

 pressed with perfect clearness by Empedocles more than 2,000 years ago 1 . 



SECTION 3. The Nature of Irritability. 



It has already been mentioned that it is in virtue of its inherent 

 disposition and special structure that an organism is able to produce from 

 the same sources of energy all the varied manifestations which are 

 characteristic of its own species and of life in general. In the same way, 

 the kind of work which a machine performs is not affected by the amount, 

 or even by the quality of the coal it consumes, provided only that a sufficient 

 supply of energy is rendered available. The nature of its component 

 parts, and the manner in which they are related to one another, are the 

 all-important factors, and the tension of the same coiled spring may either 

 direct the regular mechanism of a clock or evolve a tune from a musical 

 box. Similarly, the results which the liberation of chemical or physical 

 \ energy may produce in any given protoplast are entirely dependent upon 

 the inherent specific characters of the latter. 



In addition, therefore, to a knowledge of the sources of energy 2 , we 

 require to ascertain the nature and character of the mechanism to which 

 this energy is applied, before we can understand how the causes at work 

 produce the observed mechanical results. In the living organism not 

 only has the nature of its mechanism to be ascertained, but also the 

 processes by which it may obtain energy, for the liberation of energy 

 may produce much more varied results in a living mechanism than in an 

 ordinary machine. Thus an organism may induce, accelerate, inhibit, or 

 modify any given vital process, and indeed in this manner it acts as its 

 own essential regulatory mechanism by causing the different vital processes 

 to be correlated with one another. When we speak of manifestations of 

 irritability as vital processes which involve a liberation of energy' 1 , we 



1 Lange, I.e., 1873, Bd. I, p. 23. 



2 See Pfeffer, Energetik, 1892, and also vol. ii of this book. 



J Ostwald (Ber. d. Sachs. Ges. der Wissenschaften, Leipzig, 1894, p. 338) speaks of ' liberation 

 of energy ' in a more special sense as applying only to what takes place when a new chemical action 

 is induced. An acceleration of a chemical reaction he regards as a katalytic phenomenon. It is 

 frequently impossible to determine to which of these a given physiological reaction is due. Indeed, 

 it is not improbable that these two headings are not broad enough to include all the different means 

 by which physiological reactions are produced. 



