162 BOTANY 



quantitatively with the known operations of the laws of gravity. 

 Living substance is dominated by the operation of stimuli. Irrita- 

 bility is its most important attribute, for it is irritability alone that 

 renders possible what we call life. 



By irritability is meant the undoubted, though not fully under- 

 stood, connection between external stimuli and the response of a 

 living organism. The disproportion that may exist between a 

 cause and its ultimate effect is plainly apparent in a steam engine 

 in motion or in the firing of firearms. The slight pressure of the 

 finger in firing a cannon has as little correspondence, either quantita- 

 tively or qualitatively, with the destructive effect of the shot, as the 

 small effort necessary to open the throttle-valve of a locomotive to 

 the continuous motion of a heavily-ladened goods train. The opening 

 of the valve of an engine before the steam is up has no effect ; it 

 is only when, by this process, the compressed steam is liberated that 

 it is followed by such enormous results. In the engine the connection 

 between the cause and its effect is known ; in the effects of stimuli on 

 protoplasm this connection is not apparent, for in the protoplasm 

 the intermediate processes remain invisible to the eye, even when 

 aided by the best microscope. There is, however, no occasion for 

 the supposition that the connection between the stimulating cause 

 and its effect on the protoplasm is accomplished by processes which 

 are otherwise foreign to the protoplasm itself, and which are called 

 into existence only under the influence of a special force, the vital force. 

 It was formerly thought necessary to ascribe not only all indications 

 of life, but even all the transforming processes carried on within 

 animate objects, to the effects of a special vital force or principle. 

 Now, however, the conception of the vital processes has become so 

 modified as no longer to require the supposition of such a special 

 vital force ; while the impossibility of explaining the manifold variety 

 of their manifestation by the action of a single force, and the advances 

 made in chemistry (c/. p. 5), have shown the futility of such a 

 supposition. 



Although, at the present time, the existence of a special, in- 

 dependent vital force is denied by Physiology, and only such 

 agencies are accepted as are inherent in the substance of an organ- 

 ism itself, still we must at the same time take account of such a 

 vital force in so far as it may be regarded as the expression of a 

 living substance, endowed with a peculiar, internal structure, which 

 is in some way so constituted that certain actions and conditions 

 are followed by definite vital processes. It is, then, this peculiar 

 quality of irritability that distinguishes living protoplasm from other 

 bodies, and which constitutes the fundamental distinction between 

 living and dead protoplasm. Such a view is, however, not con- 

 trary to accepted ideas ; simple chemical bodies, indeed even 

 chemical elements, such as sulphur, phosphorus, etc., exist in different 



