the slow growth of medical knowledge it has become more and more plain that physics, chemistry, and biology are distinct sciences, with methods of their own and inductions of their own; each of the latter terms in the series using the results of its predecessors, and adding new results of its own. Although life is a structure built up of physical and chemical facts, yet to the building, to the arrangement, to the ordering of those facts, there goes something that neither physics nor chemistry can explain, any more than algebra can explain the behaviour of a magnet. To strive to interpret the series of events which make up the life of an animal in terms of chemical change (metabolism), or of conservation or expenditure of energy, is an endeavour which will fail.'
As the brute creation, as well as human beings, share in a physical organization which expresses each variety of life, there is not the same sharply-dividing line between the various categories of animal life as there is between organic and inorganic Nature. Biogenesis, or life generated by life, is the distinctive feature of organic Nature. We are linked to living creatures of higher or
