112 THE THEORY OF EVOLUTION 



may be excited and rendered possible, then can they 

 only be regarded as ' impulsions/ as building material 

 and sources of energy ; they form, as is very significantly 

 expressed, ' stimuli ' i.e. impulses and material for 

 carrying on the life of the organism. The capacity of 

 the living body to respond to an external impulse 

 with vital expressions, such as growth, formation of 

 fruit, or attractive or repulsive movements, is called 

 ' excitability ' (Reizbarkeit) . 



1 Excitability/ taken in this sense, signifies thus 

 as much as vital capability in general and is the peculiar 

 mode of reaction of all organisms in response to external 

 influences in contrast to the behaviour of inorganic 

 bodies under like influences. We are not alone in 

 this conception : it is put forward and established by 

 very eminent biologists, such as Pfeffer, Strasburger, 

 0. Hertwig, Sachs, and others. 



In the ' Lehrbuch der Botanik ' of Strasburger, 

 Noll, Schenk, and Schimper ! so extensively used in 

 the German high schools, there is the following defi- 

 nition of ' excitability ' : ' It is shown thereby that ex- 

 ternal or internal impulses given to the living organism 

 act as dissociating stimuli and induce activities 

 which it effects with means over which it has control 

 or which it is capable of obtaining, and in a manner 

 determined by its construction and by its needs. Even 

 in the smallest and simplest organism of which we 

 know, the vital processes depend on such stimuli/ 



1 Lehrbuch der Botanik fur Hochschulen, Jena, 1900, p. 4. 



