It is clear, then, that the mechanical theory of evolution can only maintain itself by obliterating the distinction between mechanical juxtaposition and chemical combination. The obstacles which stand in the way of this obliteration, at the outset, are two. First, the behaviour of a chemical compound bears no resemblance to the behaviour of its constituents when separate. Next, the laws of the compound cannot be deduced or exhibited as consequences of the laws of the separate elements. To these two objections it may be replied, first, that though the compound bears no resemblance to its separate constituents, the character of every aggregate must be determined by that of its component parts; and, next, that with more knowledge we shall come to see the way in which the laws of the separate components generate the law of the whole. Perhaps, by way of illustration, we may employ an analogy. A number of bricks can be placed on one another to form a cube; a number of cannon balls will form a pyramidical pile. The aggregate of bricks resembles in shape the separate bricks; the aggregate of balls does not resemble a ball in shape. Yet the pyramidical shape of the pile of cannon balls is as certainly determined by the shape of the separate balls, as the cubical shape of the heap of bricks is determined by that of the separate bricks. Now, we do not know the geometrical structure of chemical atoms; but, on this analogy, it is reasonable to suppose that, if we did, we should see at once that the structure of a chemical compound is dependent on, though different from, that of its elements. So too in sociology, the aggregate, society, is not a human being, but the character of any given society is determined by the character of its individual members. This last illustration, however, brings us to a fresh difficulty in the way of the mechanical theory. As is observed in the remarks quoted previously from Monsieur Bernard, the peculiar characteristic of those more intimate combinations which form the subject-matter of chemistry, physiology, and sociology is that in them the combining elements reveal properties which were not perceptible in them previous to their combination. It may be true that the character of these more intimate combinations is determined by the properties of their constituents, but it is by the properties which they reveal when in combination, not by those which are manifest in them when uncombined. Therein lies the difference between mechanical compounds and chemical combinations; and it is that difference which the mechanical theory does not account for. The more intimate, chemical, physiological, and sociological combinations take place in virtue of properties which require the combination to reveal them. In sociology it is not the juxtaposition of individual men, but their co-operation, which makes a society. In chemistry, the formation of chemical compounds implies the affinity of the elements.