with the yelk-sack only by a narrow duct. These changes through which the general structure of the embryo is marked out with slowly-increasing precision, are paralleled in the evolution of each organ. The heart is at first a mere aggregation of cells, of which the inner liquify to form the cavity, while the outer are transformed into the walls; and when thus sketched out, the heart is indefinite not only as being unlined by limiting membrane, but also as being but vaguely distinguishable from the great blood-vessels: of which it is little more than a dilatation. By and by the receiving portion of the cavity becomes distinct from the propelling portion. Afterwards there begins to be formed across the ventricle, a septum, which, however, is some time before it completely shuts off the two halves from each other; while the later-formed septum of the auricle remains incomplete during the whole of fœtal life. Again, the liver commences as a multiplication of certain cells in the wall of the intestine. The thickening produced by this multiplication “increases so as to form a projection upon the exterior of the canal;” and at the same time that the organ grows and becomes distinct from the intestine, the channels which permeate it are transformed into ducts having clearly-marked walls. Similarly, by the increase of certain cells of the external coat of the alimentary canal at its upper portion, are produced buds from which the lungs are developed; and these, in their general outlines and detailed structure, acquire distinctness step by step. Changes of this order continue long after birth; and, in the human being, are some of them not completed till middle life. During youth, most of the articular surfaces of the bones remain rough and fissured—the calcareous deposit ending irregularly in the surrounding cartilage. But between puberty and the age of thirty, the articular surfaces are finished off by the addition of smooth, hard, sharply-cut “epiphyses.” Thus we may say that during Evolution, an increase of definiteness continues long after there ceases to be any appreciable increase of heterogeneity. And, indeed, there is reason to think that those structural modifications which take place after maturity, ending in old age and death, are modifications of this nature; since they result in a growing rigidity of structure, a consequent restriction of movement and of functional pliability, a gradual narrowing of the limits within which the vital processes go on, ending at length in an organic adjustment too precise—too narrow in its margin of possible variation to permit the requisite adaptation to external changes of condition.