‘Recte ponitur’, said Francis Bacon, ‘vere scire esse per causas scire.’ The maxim of the great founder of modern inductive science has been the lode-star of biology in the past, and is still its watchword to-day. By exact observation and crucial experiment, utilizing every canon of induction, the activities of the living organism are to be brought under wide general laws of causation, which will be, in the first instance, physiological laws--of response to stimuli, of metabolism, and of growth: by means of these laws predictions can be made, and verified as often as we please. But no bar can legitimately be set to the scope of human inquiry; the thought process will not rest here, and ultimately it may be possible to state the widest generalizations of biology in chemical and physical, and these again in purely mechanical terms. The maintenance and evolution of form in the individual, as well as the larger evolution of form in the race, become but the final terms in a far vaster cosmic process, from ‘homogeneity to heterogeneity’. The idea is, of course, perfectly familiar: it is the analysis of purely physical causes, carried to its extreme limit. Phenomena are thought out in terms not of origins merely, but of one origin, and that one origin is the only mystery that remains. This unification of the sciences has always been and must still remain the dream and the faith and the inspiration of the scientific man, and could such an edifice of the intellect ever be realized, the task of science would have been completed. Only when this purely deterministic method has been pushed as far as it will go does science leave off; only where science leaves off does philosophy begin. There is an order of time, and there is an order of thought. Science works in the order of time, and necessarily so: for although science can never say what constitutes the invariable link between antecedent and consequent which it terms causal, yet it rightly speaks of the first as cause, determining the second as effect, since it is its function to predict from the past which is known to the future which is not. But the outlook of philosophy is different. Dissatisfied with the endless regress of cause and effect, sceptical of first causes and original homogeneities, out of which by no conceivability could any heterogeneity have ever been developed, philosophy looks to the end. The activities of living organisms at least appear to be directed to an end; they are apparently purposive, and it is this purposiveness which lends to biology, though built on the fundamental conceptions of chemistry and physics, peculiar features of its own, and is, of course, answerable for the teleological language which biologists so frequently employ. And by a knowledge of the end, the view of science, to which _qua_ science it cannot too rigidly confine itself, will doubtless be supplemented and enlarged.