general vigour of the {119} system, good health, easy circumstances, are all in favour of mental improvement, provided the force thus made available can be reserved and devoted to that end. Thus the two leading conditions of the plastic process are Frequency of Repetition, and Mental Concentration. For practical purposes, these are all that we need to consider, at least as regards the same individual. We have no art or device for training either body or mind but what is comprised under one or other of these heads. There are methods of superseding the labour of new acquirement, by adapting existing acquirements to new cases; but no means can be assigned for the original construction of adhesive links, apart from these two circumstances. Still, in a large and exhaustive view of the Retentive power of the mind, we should not omit to allow for the differences between one mind and another in respect of Natural Aptitude for acquiring. When two persons engaged in the same lesson, for equal periods of time, and with about equal concentration of mind, make very unequal progress, we must admit a difference in natural or constitutional plasticity on that particular subject. Sometimes we find extraordinary progress made in acquisition generally; the same person excelling in languages, in sciences, in practical arts, and in fine arts. More commonly, however, we find an aptitude for some subject in particular, combined with deficiency in other things. One person has great mechanical acquirements, another lingual, and so on. The first case is sufficiently common to justify the assumption of degrees of acquisitive or plastic aptitude on the whole, or a variety in the cerebral endowment corresponding to the adhesion of trains of actions and ideas that have been more or less frequently brought together. If the differences among human beings are not so broad as to make this apparent, we may refer to the differences between the lower animals and man. The animals have the power of acquiring, but so limited is that power in comparison with human beings, that people have often doubted its existence.