We have seen how zoology has been led back into physiological channels of research, and how the study of bacteria is opening up some of the deepest problems of the reaction of living things to environmental stimuli, and just as the various branches of these sciences interlace and influence one another, so all of them, in recent years, have been coming into contact with the inorganic sciences of chemistry and physics. One of the noteworthy features of science in all its branches in recent years has been the tendency of subjects which were at one time regarded as distinct to come together again and to find that the problems of each can only be successfully attacked by the co-operation of the others. In their earlier days the biological sciences were in most respects far removed from chemistry and physics; it was recognized, of course, that organisms were in one sense at least physico-chemical mechanisms, consisting of chemical elements and subject to the fundamental laws of matter and energy. With the advent of the theory of evolution this conception of the organism as a mechanism took more definite shape, and among many biologists the belief was held that in no very long time all the phenomena of life would be explicable by known physico-chemical laws. Hence arose the scientific materialism which was so widespread in the years following the general acceptance of Darwin's theory. It was recognized, of course, that our knowledge of organic chemistry was at the time entirely inadequate to place this belief upon a proved scientific basis, but the expectation of proving it gave a great impetus to the study of the physical and chemical phenomena of life. This attempt was still further stimulated by the investigation of the factors controlling development referred to in a preceding paragraph, for it is evident that to a great extent at least these factors are chemical and physical in nature. And concurrently, the great advances in organic chemistry, resulting in the analysis and in many cases in the artificial synthesis of substances previously regarded as capable of production only in the tissues of living organisms, made possible a much more thorough investigation of the chemical and physical basis of vital phenomena. The result of this has been that to a quite considerable extent the factors, hitherto mysterious, which control the fertilization, division, and differentiation of the egg, the digestion and absorption of food, the conduction of nervous impulses, and many of the changes undergone in the normal or pathological functioning of the organs and tissues, can be ascribed to chemical and physical causes which are well known in the inorganic world.