writers are evidently conscious that they are identifying two disparates, in order to be able to make use of the one or the other, according to circumstances ; for this end they are obliged to keep the identification constantly before their readers mind.—
Now Causality, as the director of each and every change, presents itself in Nature under three distinct forms: as causes in the strictest acceptation of the word, as stimuli, and as motives. It is just upon this difference that the real, essential distinction between inorganic bodies, plants, and animals is based, and not upon external, anatomical, let alone chemical, distinctions.
A cause, in its narrowest sense, is that upon which changes in the inorganic kingdom alone ensue : those changes, that is to say, which form the theme of Mechanics, Physics, and Chemistry. Newton's third fundamental law, "Action and reaction are equal to one another," applies exclusively to this cause, and enunciates, that the state which precedes (the cause) undergoes a change equivalent to that produced by it (the effect). In this form of causality alone, moreover, does the degree of the effect always exactly correspond to the degree of the cause, so as to enable us accurately to calculate the one by means of the other.
The second form of causality is the stimulus ; it reigns over organic life, as such, i.e. over plant life and the vegetative, that is, the unconscious, part of animal life. This second form is characterized by the absence of the distinctive signs of the first. In it accordingly action and reaction are not equal, nor does the intensity of the effect by any means correspond throughout all its degrees to the intensity of the cause ; in fact, the opposite effect may even be produced by intensifying the cause.
The third form of causality is the motive. Under this form causality rules animal life proper : that is, the exterior,
