vii.] ON THE PHYSICAL BASIS OF LIFE. 123 



rightly name the higher faculties, arc not excluded from 

 this classification, inasmuch as to every one but the subject 

 of them, they are known only as transitory changes in 

 the relative positions of parts of the .body. Speecb, 

 gesture, and. every other form of human action arc, in 

 the long run, resolvable into muscular contraction, and 

 muscular contraction is but a transitory change in the 

 relative positions of the parts of a muscle. But the 

 scheme which is lanre enough to embrace the activities 

 of the hi o-h est form of life, covers all those of the lower 

 creatures. The lowest plant, or animalcule, feeds, grows, 

 and reproduces its kind. In addition, all animals manifest 

 those transitory changes of form which we class under 

 irritability and. contractility; and, it is more than 

 probable, that when the vegetable world is thoroughly 

 explored, we shall find all plants in possession of the 

 same powers, at one time or other of their existence. 

 I am not now alluding to such phenomena, at once 

 rare and conspicuous, as those exhibited by the leaflets 

 of the sensitive plant, or the stamens of the barberry, 

 but to much more widely- spread, and, at the same time, 

 more subtle and hidden, manifestations of vegetable 

 contractility. You are doubtless aware that the common 

 nettle owes its stinging property to the innumerable stiff 

 and needle-like, though exquisitely delicate, hairs which 

 cover its surface. Each stinging-needle tapers from a 

 broad base to a slender summit, which, though rounded 

 at the end, is of such microscopic fineness that it readily 

 penetrates, and breaks off in, the skin. The whole hair 

 consists of a very delicate outer case of wood, closely 

 applied to the inner surface of which is a layer of semi- 

 fluid matter, full of innumerable granules of extreme 

 minuteness. This semi-fluid lining is protoplasm, which 

 thus constitutes a kind of bag, full of a limpid liquid, 

 and roughly corresponding in form with the interior of 



