If we do not restrict the term "life" to organisms properly so-called, and take it only as a function of plasm, we may speak in a broader sense of the life of crystals. This is seen especially in their growth, the phenomenon which Baer regarded as the chief character of all individual development. When a crystal is formed in a matrix, this is done by attracting homogeneous particles. When two different substances, A and B, are dissolved in a mixed and saturated solution, and a crystal of A is put in the mixture, only A is crystallized out of it, not B; on the other hand, if a crystal of B is put in, A remains in solution and B alone assumes the solid crystalline form. We may, in a certain sense, call this choice _assimilation_. In many crystals we can detect internally an interaction of their parts. When we cut off an angle in a forming crystal, the opposite angle is only imperfectly formed. A more important difference between the growth of crystals and monera is that the former only grow by _apposition_, or the deposit of fresh solid matter at their surface; while the monera grow, like all cells, by _intussusception_, or the taking of new matter into their interior. But this difference is easily explained by their difference in consistency, the crystal being solid and the plasm semi-fluid. Moreover, the difference is not absolute; there are intermediary stages between apposition and intussusception. A colloid globule suspended in a salt solution in which it is not dissolved may grow by intussusception. It was once the custom to restrict sensation and movement to animals, but they are now recognized to be present in nearly all living matter. They are, in fact, not altogether lacking in crystals, as the molecules move in crystallization in definite directions, and unite according to fixed laws; they must, therefore, also possess sensation, as we could not otherwise understand the attraction of the homogeneous particles. We find in crystallization, as in every chemical process, certain movements which are unintelligible without sensation--unconscious sensation, of course. In this respect, also, then, the growth of all bodies follows the same laws (_cf._ chapters xiii. and xv.).