Page:Scientific Memoirs, Vol. 1 (1837).djvu/253

Rh 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 activity, 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 operations: 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 influence appears as a change in its physical properties, for instance in its extension; 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 influence 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 organic 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 relation 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, originates 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 necessarily 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 existence 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