Thus the child appears, as soon as it is capable of recognizing any source of its impressions, to regard an object as situated in space, an event as happening in time, circumstances which have occurred together as likely to occur together again. But Kant's own account was defective in leaving this _à priori_ element of our knowledge unexplained, or, at least, in attempting no explanation of it. The mind, according to him, is possessed of certain Forms and Categories, which shape and co-ordinate the impressions received from the external world, being as necessary to the acquisition of experience, as experience is necessary to eliciting them into consciousness. But here his analysis ends. He does not ask how the mind comes to be possessed of these Forms and Categories, nor does he satisfactorily determine the precise relation in which they stand to the empirical elements of knowledge. When studying his philosophy, we seem indeed to be once more receding to the mysterious region of Innate Ideas. But the mystery is removed at least several stages back, if we apply to the solution of these mental problems the principle of Heredity, which has recently been found so potent in clearing up many of the difficulties connected with external nature. What are the "Innate Ideas" of the older philosophers, or the Forms and Categories of Kant, but certain _tendencies_ of the mind to group phenomena, the "fleeting objects of sense," under certain relations and regard them under certain aspects? And why should these tendencies be accounted for in any other way than that by which we are accustomed to account for the tendency of an animal or plant, belonging to any particular species, to exhibit, as it developes, the physical characteristics of the species to which it belongs? The existence of the various mental tendencies and aptitudes, so far as the individual is concerned, is, in fact, to be explained by the principle of hereditary transmission. But how have these tendencies and aptitudes come to be formed in the race? The most scientific answer is that which, following the analogy of the theory now so widely admitted with respect to the physical structure of animals and plants, assigns their formation to the continuous operation, through a long series of ages, of causes acting uniformly, or almost uniformly, in the same direction--in one word, of Evolution. This explanation may have its difficulties, but it is at any rate an attempt at a natural explanation where no other such attempt exists, and it has the merit of falling in with the explanations of corresponding phenomena now most generally accepted amongst scientific men in other departments of knowledge.