In addition to these outward sources of health-disturbance, which constitute the disease of mankind, there are other and more subtle and internal causes which complicate the problems of human happiness. Thus, for example, each individual inherits from his parents, and through them from his more remote ancestors, a certain physical constitution. This constitution, whilst no doubt liable to modifications, yet determines wholly or in greater part the physical life of the being possessing it. We frequently speak of persons as suffering from inherited weakness, and this inherited weakness becomes the ‘transmitted disease’ of the physician. Each individual, therefore, may be viewed as deriving his chances of health, or the reverse, from a double source—namely, from the constitution he has inherited and from the surroundings which make up the life he lives and pursues. It is the aim and object of sanitary science to deal as clearly and definitely as possible with both sources of health and disease. In the first instance, Hygiene, or the science of health, devotes attention to the surroundings amid which our lives are passed. It seeks to provide us with the necessary conditions of life in a pure condition. It would have us breathe pure air, consume pure food, avoid excess of work, strike the golden mean in recreation, and harbour and conserve the powers of old age, so as to prolong the period of life and secure a painless death. In the second aspect of its teachings, this important branch of human knowledge would teach us that with an inherited constitution of healthy kind we should take every means of preserving its well-being; and when on the other hand an enfeebled and physically weak frame has fallen to our lot, the teachings of health-science are cheering in the extreme. Even when an individual has been born into the world, handicapped, so to speak, in the struggle for existence by physical infirmity and inherited disease, health-science is found to convey the cheering assurance that it is possible, even under such circumstances, to prolong life, and secure a measure of that full happiness which the possession of health can alone bestow. In illustration of this latter remark, we might cite the case of a person born into the world with a consumptive taint, or suffering from inherited tendencies to such diseases as gout, rheumatism, insanity, &c. Vital statistics prove beyond doubt, in the case of the consumptive individual, that if his life be passed under the guidance of health laws, if he is warmly clad, provided with sufficient nourishment, made to live in a pure atmosphere, and excess of work avoided, he may attain the age of thirty-six years without developing the disease under which he labours, and once past that period, may reasonably hope to attain old age.