Those physiological functions of the human body that appear to be clearly marked off are really not completely demarcated. Take as example the excretory action of the skin, lungs, and the renal organs. The lungs get rid especially of carbon dioxide; the skin of water; the renal organs of the products of nitrogenous decay. But each of these organs also eliminates those products which are eliminated by the other two. Thus the lungs, whilst they get rid principally of carbon dioxide, also get rid of water in the form of steam and of nitrogenous matter. The skin gives off a certain quantity of carbon dioxide and nitrogen excreta. And the renal organs also eliminate all three of the chief forms of excretory matter. When any one of these three organs is not functioning at its best, extra work is thrown upon the others, and in some extreme cases this metastasis, or transference of function, is very remarkable. Thus an ulcer in the human body has been known to secrete milk. Try to realise at least something of what all this means. It is no longer possible to mark off clearly the various domains of science. Science is one, for it is the study of nature, and nature is one. In every branch of our knowledge that daily grows more unified, the transitions are found to be innumerable and the gradations infinitesimal. Our chemical groups, our geological rocks and strata, our inorganic and organic kingdoms, our plants and animals, our classes, orders, genera, species, all are seen to be artificial.