754 PLANT RESPONSE 



after which the impressed periodicity is lost. The autum- 

 nal periodicity of trees which grow in temperate climates, 

 where the leaves are habitually shed at the approach of 

 winter, persists for a few or for numerous years when the 

 plant is transferred to a warm climate. Certain species of 

 bacteria, again, can be made to exhibit an artificial change of 

 characteristics which will persist during many generations, 

 even on the return of the organism to normal conditions ; 

 but even after this degree of hereditary persistence of effect 

 has been exhibited, these induced varieties are liable to 

 undergo reversion to their original type. It is thus seen 

 that the persistence of hereditary characteristics is merely 

 relative. Had it been absolute, the species itself would have 

 been immutable. A given rhythm only persists so long as 

 those circumstances which originally occasioned it persist, 

 and during a certain period afterwards ; but this after- or 

 hereditary-effect must, in the non-continuance of the original 

 periodically exciting cause, prove, however long maintained, 

 to be but transitory, like the ultimate arrest of a pendulum 

 when its vibrational energy is exhausted. 



The phenomenon of life, then, introduces no mystical 

 power, such as would in any way thwart, or place in 

 abeyance, the action of forces already operative. In the 

 machinery of the living, as in that of the non-living, we 

 merely see their transformation, in obedience to the same 

 principle of conservation of energy as obtains elsewhere ; 

 and it may be expected that, in proportion as our power of 

 investigation grows, the origin of each variation of the living 

 organism will be found more and more traceable to the 

 direct or indirect action upon it of external forces, the 

 element of chance being thus progressively eliminated, as the 

 definite sequence of cause and effect comes to be perceived 

 with an increasing clearness ; and only, I venture to think, 

 as this is worked out, can we learn to apprehend fully the 

 true significance of the great Theory of Evolution. 



