10 



ory 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 contrac- 

 tion., 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 activ- 

 ities 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 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 contrac- 

 tility. 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 pen- 

 etrates, 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, 



