If the mystery of life, lying beneath and behind organic behaviour, be said to baffle scientific interpretation, this is because it suggests ultimate problems with which science as such should not attempt to deal. The final causes of vital phenomena (as of other phenomena) lie deeper than the probe of science can reach. But why is this sense of mystery especially evoked in some minds by the contemplation of organic behaviour, by the study of life? Partly, no doubt, because the scientific interpretation of organic processes is but recent, and in many respects incomplete. People have grown so accustomed to the metaphysical assumptions employed by physicists and chemists when they speak of the play of crystalline forces and the selective affinities of atoms, they have been wont for so long to accept the “mysteries” of crystallization and of chemical union, that these assumptions have coalesced with the descriptions and explanations of science; and the joint products are now, through custom, cheerfully accepted as natural. Where the phenomena of organic behaviour are in question, this coalescence has not yet taken place; the metaphysical element is on the one hand proclaimed as inexplicable by natural science, and on the other hand denied even by those who talk glibly of physical forces as the final cause of the phenomena of the inorganic world. So much reference to the problems which underlie the problems of science seems necessary. It is here assumed that the phenomena of organic behaviour are susceptible of scientific discussion and elucidation. But even assuming that an adequate explanation in terms of antecedence and sequence shall be thus attained by the science of the future, this will not then satisfy, any more than our inadequate explanations now satisfy, those who seek to know the ultimate meaning and reason of it all: What makes organic matter behave as we see it behave? what drives the wheels of life, as it drives the planets in their courses? what impels the egg to go through its series of developmental changes? what guides the cells along the divergent course of their life-history? These are questions the ultimate answers to which lie beyond the sphere of science--questions which man (who is a metaphysical being) always does and always will ask, even if he rests content with the answer of agnosticism; but questions to which natural science never will be able, and should never so much as attempt, to give an answer. Enough has now been said to show that organic behaviour is a thing _sui generis_, carrying its own peculiar marks of distinction: and further, that, for science, this is just part of the constitution of nature, neither more nor less mysterious than, let us say, crystallization or chemical combination. But associated and closely interwoven with all that is distinctively organic there is much which can to some extent be interpreted in terms of physics and chemistry.