Such objections as these must, I think, arise in the mind of every one who ponders, for the first time, upon the conception of a single physical basis of life underlying all the diversities of vital existence; but I propose to demonstrate to you that, notwithstanding these apparent difficulties, a threefold unity—­namely, a unity of power, or faculty, a unity of form, and a unity of substantial composition—­does pervade the whole living world.

No very abstruse argumentation is needed, in the first place, to prove that the powers, or faculties, of all kinds of living matter, diverse as they may be in degree, are substantially similar in kind.

Goethe has condensed a survey of all the powers of mankind into the well-known epigram:—­

“Warum treibt sich das Volk so und schreit?  Es will sich ernaehren
Kinder zeugen, und die naehren so gut es vermag.

* * * * *

Weiter bringt es kein Mensch, stell’ er sich wie er auch will.”

In physiological language this means, that all the multifarious and complicated activities of man are comprehensible under three categories.  Either they are immediately directed towards the maintenance and development of the body, or they effect transitory changes in the relative positions of parts of the body, or they tend towards the continuance of the species.  Even those manifestations of intellect, of feeling, and of will, which we rightly name the higher faculties, are 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.  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 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 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 phaenomena, 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