26 Plant Life and Evolution 



inevitable fluctuations in the environment may be 

 considered as the fundamental causes of that varia- 

 tion which is the beginning of any line of evolution. 

 The Effects of Stimuli. As we are very ignorant 

 of the physical structure of protoplasm we can only 

 guess at the reactions that are developed within it 

 as the results of various stimuli. While we speak 

 of the formative effects of light, heat, and other 

 extrinsic factors, it is extremely unlikely that the ef- 

 fects of these are immediate. As the results of their 

 action certain effects finally develop; but how far 

 these are the direct result of the evident stimuli, 

 and how far they are caused by others not so ap- 

 parent, we have no means of judging. It is certain 

 that a single, apparently insignificant stimulus may 

 set in motion a chain of reactions which result in 

 far-reaching effects. We might compare this to a 

 mine, which may be fired by a single spark, or by 

 a percussion cap, the final result of the explosion 

 being the annihilation of a whole town. A fern- 

 spore lies dormant in a state of physiological equi- 

 librium ; it is placed in water, and immediately there 

 is set up a series of reactions which result in its 

 germination and final development into the mature 

 plant. Light or heat may be the stimulus which 

 is necessary, but once inaugurated the succession of 

 reactions must follow. The fertilization of the egg 

 and its subsequent development into the animal of- 

 fers an equally striking instance of the far-reaching 

 results of an apparently slight stimulus. 



