investigations into that border-territory of Nature where animal and vegetable life touch, we meet with so-called monads—the Heteromita, for example—which may be referred with equal justice to either kingdom; there are organisms which we think vegetable, having characters which we call animal, and organisms which we call animal, having characters which we think vegetable; there is, in truth, no line of demarkation, but instead an insensible series of gradations, and no man can say where the one kingdom ends and the other begins. In like manner, notwithstanding the seemingly gross and palpable distinction between living and dead matter, any one who sets himself to work to find out where life begins will be hard put to it to draw a line of separation, and more hard put to it when called upon to make good his division. Man himself, much as he makes of himself, is not separated from the rest of Nature by an impassable gulf; he modifies Nature largely, it is true, but the art by which he does that is Nature; he is a part of the order thereof—the latest product of the evolution which went on for countless ages before he appeared upon earth, which is going on now in his progress, his knowledge and his moral feelings being agencies in the process, and which, for anything we know, will go on for countless ages after the earth, which he has ceased to replenish and subdue, has fallen into the condition in which the moon now is, and rolls on its solitary way through space, a cold and desert globe, the tomb of all human aspirations, sorrows, sins, and achievements. In making use, then, of the arbitrary divisions of our sciences, we ought never to lose hold of the actual unity and continuity of Nature; never to overlook the fact that there is not a single truth in any science which has not its essential relations with the truths of all sciences; never to forget that the least things and the greatest are indissolubly bound together as equally essential elements of the intimately connected and mysterious whole which we call the universe. It may seem a fanciful saying, but there is a truth in it, that you cannot utter an exclamation, strike a note on a piano, move a grain of sand from its place, without affecting the entire universe.
Now the systematic training of the mind in conformity with the order of Nature, through patient observation and careful induction, the knowledge of Nature which is got by becoming, as Bacon says, her servant and interpreter, is a tedious business. Men, therefore, have gladly shirked it; they have found it much easier to attribute phenomena to some metaphysical entity which they have created out of a mental abstraction, or to invoke a supernatural cause to account for them, than to find out the explanation. In consequence of this habit of mind, which has had large operation in the past, a body of doctrine has grown up which, having had its day, is now fast becoming effete, but which men will not willingly part with—doctrine comparable, if I may use a physiological comparison, with those organs which, like the thymus gland, have their uses at a certain stage of