THE NATURE OF IRRITABILITY 21 



speak of isogenetic or heterogenetic induction *. The nature of any given 

 irritability, and hence also the response made to a stimulus, are, as a matter 

 of fact, always influenced by previous stimulation and induction of cither 

 general or special character. As in the case of every vital phenomenon, 

 each manifestation of irritability is dependent for its production upon 

 a variety of factors, of which the actual stimulus forms but one. 



Both real and imaginary machines afford instances of the manifold 

 nature of the response possible, and of the self-regulatory action of which 

 the different parts are capable. For in many machines, as well as in 

 plants, the different parts of the entire mechanism may act as regulatory 

 stimuli to one another when the whole is at work, so that the ultimate 

 result has a very complex origin. 



There can be no doubt that the wonderful correlation existing 

 betwetn the different vital activities is directly clue to the complex 

 interactions between the various forms of irritability and the excitatory 

 stimuli. 



A knowledge of the essential principles governing the endless variety 

 of possible phenomena of irritability can hardly be obtained by reference to 

 those manifestations which are of peculiar and special character, although 

 the same general principles which have already been established as applying 

 to irritable processes in general, may in these cases also be recognized. The 

 remarkable and complicated phenomena of accommodation are examples 

 of special and peculiar manifestations of irritability, and are in general 

 to be regarded as due to the gradual transition to the new condition of 

 equilibrium which is established by means of the self- regulatory vital 

 activity. Thus it is by means of such accommodatory reactions that 

 plants may gradually be accustomed to doses of poison that originally 

 would have proved fatal, or that a branch may be enabled to resist 

 mechanical strains which would have broken it if suddenly applied. 

 Another example is afforded by the fact that plants which receive a 

 deficient supply of oxygen or nutriment pass after a time into a special 

 condition in which the different vital processes are performed more slowly 

 and economically. 



These examples suffice also to show that it is not always immaterial 

 whether the change of conditions takes place instantaneously or only 



1 These words are used in a different sense from that in which Noll uses them (Heterogene 

 Induction, 1892, p. 14). He fails to realize the general character of heterogenetic induction or the 

 universal importance of stimulatory induction (see Pfeffer, Die Reizbarkeit, &c., 1893, P- 22 )- It is 

 neither justifiable nor correct to apply this term only to certain specially noticeable examples of 

 heterogenetic induction, and neglect all the commoner and more general forms of stimrtlatory induc- 

 tion. Moreover, it is impossible to draw a sharp distinction between the general and special essential 

 conditions, for in any irritable manifestation the action of an additional and unnecessary stimulus 

 becomes essential for the production of the result in its new and modified form. These remarks 

 apply also to the ideas put forward by Herbst, Biol. Centralbl., 1894, Bd. xiv, p. 732. 



