THEIR LIFE AND AFFINITY. 24r 



of a susceptibility or capacity to be efFected by those external causes. 

 This property, so far as it is conducive to the excitement of organic ac- 

 tivity, has been named irritability, but always without annexing to the 

 word a sufficiently precise idea. We think it therefore, not superfluous 

 to illustrate it by some examples ; when, for instance, a body is to be 

 put in motion by an impulse, it is necessary that it possess mobility, or 

 the capacity to be put in motion. It is the same with chemical opera- 

 tions : in order that a body be acted upon and decomposed by another 

 body, it is requisite that the former be susceptible of this chemical 

 action. The case is the same if an organism as a whole, comprehended 

 in an enduring form, is to be affected by external influences ; except that 

 we must here distinguish whether the activity called forth by this influ- 

 ence appears as a change in its physical properties, for instance in its ex- 

 tension ; or as a change in its own organic activity, its formation ; or in 

 the mutual relation of its single parts. In the former case we name 

 this property a physical recepti\'ity, in the second irritability, and the 

 exciting power a stimulant; from which it is clear that the same in- 

 fluence can act both as a mere physical power and as a stimulant : heat, 

 for instance, can expand a body, and at the same time quicken its or- 

 ganic formation or growth ; in the latter case it acts as a stimulant. 

 Hence it follows that the irritability of the plant stands in the same re- 

 lation to animal sensibility, as its own physical receptivity stands to its 

 irritability. While the plant therefore, from being indebted for its own 

 movements to the influence of external causes, approaches more nearly 

 to universal nature, and is therefore further separated from animal life, 

 to M'hich it approximates again in the inclining of the stamina towards 

 the stigma, this movement, though independent of its own will, origi- 

 nates in an attraction inherent in the plant itself. 



Having hitherto been occupied in considering the influence of the 

 organization of the earth upon plants, it necessarily follows that we 

 should consider the influence which the vegetable kingdom exercises 

 upon the life of the earth ; for even though we should not be inclined 

 to consider that the origin of the vegetable kingdom in general neces- 

 sarily marks an important epoch in the formation of the earth, — as 

 for instance, in the development of the plant, the production of a single 

 organ (as the flower) from a particular influence is to the whole 

 plant, — yet the transformation of vast masses of vegetable substances 

 into strata of pit and Bovey coals, into strata of turf and of vegetable 

 mould, and particularly the influence of living vegetation upon the 

 surface of the earth and atmosphere, are objects too striking to pass 

 unnoticed. In the latter point of view it is particularly worthy of 

 remark, that the origin of brooks and streams is owing to the exis- 

 tence of woody mountains, and their greater attraction of atmospheric 

 vapours; wherefore we often see streams dried up, on account of 

 the destruction of the forests in which their sources lay ; a . cir- 



