INTRODUCTION 255 



Many of the phenomena which we have become acquainted with suggest 

 comparisons not only between organisms and complex conditions in the non- 

 living world, but also in another direction. We can distinguish internal and 

 external causes in vegetable phenomena ; only when these work in harmony is 

 development or any other activity possible (C. Bernard, 1878). Take the 

 bean, for example. Germination takes place only if certain external condi- 

 tions be fulfilled ; there must be certain materials present in the medium in 

 which the development takes place, and also water and oxygen ; again, there 

 must be a certain temperature, and in the later stages, at least, sufficient 

 illumination. That co-operation of internal factors on the other hand is essential 

 is shown by the fact that identical external conditions can induce no develop- 

 ment in seeds which have died after prolonged keeping, but which are otherwise 

 unaltered, and further that bean plants always arise from bean seeds, while 

 from peas an entirely different type of plant arises. It would be an arbitrary 

 proceeding to assume that any one of these many causes is the chief factor in 

 the phenomenon concerned. 



The activity of a piece of machinery is also dependent on the interaction 

 of internal and external factors. Its specific activity depends on the arrange- 

 ment of its constituent parts, and it is only when these parts are co-ordinated 

 in a systematic manner that it can perform its functions properly. But if the 

 machine is to do work the external conditions must also be fulfilled — in a steam- 

 engine, for example, when a certain pressure of steam acts on the piston. Hence 

 it is frequently the custom to compare an organism with a piece of mechanism, 

 and this comparison may be carried further when the significance of each in- 

 dividual factor in the performance of work is taken into account. In the plant, 

 as in the machine, we may distinguish certain factors which directly provide it 

 with energy to carry out the work, and others which may be looked upon as 

 merely liberating energies. The opening of the stop-cock which permits the 

 steam to enter the piston-box is a liberator of this type ; so is the pulling of the 

 trigger of a rifle. In neither case does the necessary pressure of the finger bear 

 any direct relation to the work done by the machine ; it only releases a pre- 

 existing energy and allows it to perform work. The work in the one case is 

 done by the expansion of steam, in the other, in the first instance, by the spring 

 of the rifle, and then by the explosive force of the powder. In the plant only 

 a few cases are known in which an external factor supplies directly the energy 

 required for the production of the result, e, g. the action of sunlight in carbon 

 assimilation, or the sugar in the nourishment of heterotrophic plants ; in the 

 large majority of cases the external factors merely liberate energy, that is to 

 say, act as ' stimuli ' (Pfeffer, 1893) — and the work is done by energies already 

 stored in the plant. It is very frequently the case in the plant that one released 

 movement releases another and so on, so that a whole series of reactions may 

 occur between the obvious primary release and the obvious final result, just 

 as in the case of the rifle between the pulling of the trigger and the impact 

 of the bullet on the target. The plant is, in a sense, 'loaded' and ready to 

 transform its potential energy into kinetic whenever the necessary stimulus is 

 applied. 



Another important similarity between an organism and a machine lies in 

 its power of self-regulation. Just as in a steam-engine an excessive speed is 

 reduced automatically by that very increase, so in an organism similar self- 

 regulating mechanisms occur ; compare in this relation what has been said as 

 to the production of diastase (p. 183). 



Differences between organisms and machines are, however, not wanting. 

 We must take into account, in the first place, the much more complicated nature 

 of the organism, in which respect naturally there is no fundamental difficulty in 

 making a comparison. Although we have compared the organism with a loaded 

 rifle, still we must note that this comparison gives us but a feeble idea of the 



