ON THE PHYSICAL BASIS OF LIFE 93 



faculties, are not excluded from this classification, inas- 

 much 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. Speech, gesture, and every other form of 

 human action are, 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 large enough to embrace the activi- 

 ties of the highest 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 irri- 

 tability and contractility; and, it is more than probable, that 

 w^hen 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 plants, 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 sum- 

 mit, which, though rounded at the end, is of such micro- 

 scopic 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 the 



