This change is brought about by means of a vital fluid in the body, which circulates from the moment in which the spermatozoon, or male seed, touches the female egg in the womb of the mother, until the time of our last breath. That fluid is _the blood_,--the carrier of nature's supplies to all parts of the body for the rebuilding of cells; the exact and equitable distributor in quantities of material which determines the quality of the cells. In its marvelous performance of this function, the blood is the bearer of the sole existing condition of health; namely the necessary elements of cell-building in the right proportions. This is health, and the lack thereof is disease. The demand of nature for upbuilding and rebuilding is the strongest instinctive impulse of our being; and this being so, a wrong proportion may cause the upbuilding of things which are different and disturbing to the normal organism. But, on the other hand, kindly nature exhibits an ever existent inclination to counterbalance any disturbance in the right proportion, and to bring back conditions to uniformity. We may thus justly speak of _the overwhelming healing tendency of nature_. Metabolism is, therefore, the one great dominant function of the body which, accordingly, must have our especial care. It is the blood, consequently, to which alone we can resort if we desire to assist nature in its process and tendency of balancing and healing. This again indicates that, notwithstanding the apparent great variety of _constitutional diseases, they are all practically one and the same disease. They are all disturbances of proper metabolism, by some irregularity of the quantitative or qualitative condition of the blood_. This governing truth the great physiologist, Prof. Jacob Moleschott, has formulated in the memorable words: "It is one of the chief questions which humanity must always ask of the physician: how to attain good, healthy and active blood. And, view the question as we may, all who give it serious thought, are forced by experience to acknowledge explicitly, or otherwise, that _our mental and physical capacity, and likewise the power of reproduction, are directly dependent upon our blood, and our blood on our nutrition_." VARIETY OF ORGANS. Why then, you may ask, if such unity exists, why this dissimilarity in the tissues of the respective bodily organs? How is it that a bone in its stonelike hardness is essentially the same as the infinitely tender tissues of the eye? This difference is due to and accounted for by the adaptation of certain portions of the immense accumulation of cells to diverse functions, which has necessitated the variable conformity of the supporting elements. But all of these elements are in the blood, which carries them in the necessary quantities to the different organs to which they belong and where they are utilized to replace used-up matter. I do not overlook the difficulty of grasping this idea of unity.