Science only begins on the day when we have found the simple theory which binds together all the facts at that time known, without of course prejudicing the future. As the number of acquired facts increases, if the theory in question continues to explain the new as it explained the old, the science becomes more firmly established. If we can imagine a time arriving when all the possible phenomena are known, and the existing hypothesis still explains them, nothing henceforth can overturn it, the science is completed. That is the simple case in which a theory has been victorious; but if it is contradicted by a single well-authenticated fact it must fall or become modified. The more things a theory explains in the present the more chance it has of success in the future. It is still only a matter of chances, for the theory is always at the mercy of unforeseen observation, which may rudely overthrow it. There is no theory which must not be modified constantly, at least in its details. To render it more and more general by successive improvements is the aim to be pursued. A collection of studies constitutes a science when a hypothesis has arisen already sufficiently strong to oblige us to refer to it all new acquisitions, and to compel us to see if they fortify or oppose it. It would indeed be a narrow conception if we were to consider as scientific the partisans of the theory alone; more than anywhere else discussion is fruitful in the natural sciences; and if it is necessary to be constantly preoccupied with the general ideas of the day, it is not at all necessary to adhere to them servilely. The naturalists of to-day are in possession of a formula with which we must always preoccupy ourselves; in other words, there are natural sciences. _The theory of Evolution._--This hypothesis which comes before all others is the theory of evolution. This is not the place to expound it, to go over the proofs which have been amassed to build it up, nor the criticisms which have been directed against it. It has to-day come out of the struggle victoriously. A prodigious quantity of facts, of comparative anatomy and of embryology, inexplicable without it, emerge from the chaos and constitute a whole, truly and marvellously homogeneous. Issued from the natural sciences, the doctrine of evolution now overflows them and tends to embrace everything that concerns man: history, sociology, political economy, psychology. The moralists seek, and will surely find, compromises permitting ethical laws to endure the rule of this overwhelming hypothesis.