The first weeks and months of life are the most fatal, so much so that about a fourth of the children born die within the first year, while in many towns more than half the deaths are of children under a year old. There are various explanations of this, perhaps the chief being that the helpless infant can scarcely let its wants be known, and being dependent on the care of others, is more susceptible to all sorts of accidental external influences. Hence it is that the children of the well-to-do are much less affected by disease than those of poorer parents, while illegitimate children who receive least care are much more liable to disease and death than those born in wedlock. Besides this, however, there are other explanations. The child, emerging from the protection and warmth of the uterus, undergoes a sudden transformation in its circulation and nutrition, and at the same time has to cope with physical conditions, chiefly mechanical and thermal, which are new to it. The first days and weeks of life are therefore the most trying to the child. In the child, again, the tissues are growing, and have the double duty of nutrition and growth thrown upon them. They are therefore more liable to derangement than those of adults, although they possess greater powers of restoration when damage is done. The smaller bulk of the child's body, also, renders it more susceptible to changes of temperature, so that cold will penetrate more deeply into its tissues than into those of adults, a fact which is not always remembered in the clothing of children. Besides all this, the bodies of children present, as it were, a virgin soil for the propagation of the various infective agents. The children of the poor can scarcely escape exposure to the ordinary infectious diseases, and their bodies are apparently very susceptible to these influences. A striking illustration of these statements lies in the fact that, in Glasgow, whooping-cough is the cause of more deaths in children than any other communicable disease (Russell). Tuberculosis, also, is extremely prevalent in children, and is much more liable to extend beyond its primary local seat than in adults; as if the tissues of children afforded a better nidus for the infective agent. The greatest liability to disease is in the first year of life, and it diminishes from the first year to the eighth. From the eighth till the eighteenth year the liability to disease increases, but diminishes after the latter age, reaching a minimum between twenty-four and thirty, to increase from that point onwards. The mortality by no means follows the liability to disease. It is, indeed, greatest in the first year, diminishing from that period onwards. The diminution, however, does not cease at the eighth year, but goes on till adult life, the minimum being between the ages of twenty aud forty-five. After the latter age the death-rate rises regularly but slowly.

The increase of disease from the eighth to the eighteenth years is related' to the fact that these are, in general, the years, of school-life and include the time of puberty. School, by exposure in going and coming, and sometimes by too prolonged confinement, not only renders the body less resistant, but also exposes children to infection. The period of puberty, involving serious changes in the bodily functions, is liable to induce a general weakness of the body, and also exposes females to the special affections of menstruation.

In advanced life, the body is liable to diseases which imply decadence or senility of the tissues. The bones are more brittle by reason of a diminution in their animal constituents, and hence are liable to give way under a less severe mechanical strain than are ordinary bones. Senility of the blood-vessels affects their liability to certain forms of disease, and, indeed, the diseases referred to are mainly present in persons past middle life. Affections of the brain are frequent in old persons, but nearly all of these depend on the condition of the bloodvessels, and are not primarily diseases of the brain. Cancer is very specially a disease of middle and advanced life, but not of old age. It is characterized by an excessive growth of the epithelial tissues, and it is difficult to explain how this should occur preponderatingly at an age beyond that of greatest vigour, unless on the assumption that, so long as the surrounding tissues remain vigorous, they restrain any tendency in the epithelial tissues to grow to excess.