‘“THEIR LIFE AND AFFINITY. > ~ 7 IAT 
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 receptivity, in the second irritability, and the 
exciting power a stimulant; from which it is clear that the same in- 
fluence can act both asa mere physical power and asa 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 which 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- 
