7 



classification, iiiasmucli as to every one but tlie subject of tliem, 

 they are known only as transitory changes in the relative posi- 

 tions of parts of the body. Speech, gesture, and every other 

 form of human action are, in the long run, resolvable into muscu- 

 lar 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 activities 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 veget^ifo^fi^ld is thoroughly ex- 

 plored, we shall find all plants in poJR^^s^^^f the same powers, 

 at one time or other of their existei]^H 'Pam not now alluding 

 to such phenomena, at once rare and conspicuous, as those ex- 

 hibited 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 con- 

 tractility. You are doubtless aware that the common nettle owes 

 its stinghig property to tlie innumerable stifl" 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 ofii" in, the skin. The wliole 

 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 inte- 

 rior of tlie hair which it fills. When viewed with a sufiiciently 

 liigh magnifying power, the protoplasmic layer of the nettle hair 

 is seen to be in a condition of unceasing activity. Local con- 

 tractions of the whole thickness of its substance pass slowly and 

 gradually from point to point, and give rise to the appearance of 

 progressive waves, just as the bending of successive stalks of 

 corn by a breeze produces the apparent billows of a corn-field. 

 But, in addition to these movements, and independently of them, 

 the granules are driven, in relatively rapid streams, through 

 channels in the protoplasm which seem to have a considerable 

 amount of persistence. Most commonly, the currents in adjacent 

 parts of the protoplasm take similar directions ; and, thus, there 



