"This terrestrial evolutionary working of Nature from Matter to Mind and beyond it has a double process: there is an outward visible process of physical evolution with birth as its machinery, -for each evolved form of body housing its own evolved power of consciousness is maintained and kept in continuity by heredity, there is, at the same time, an invisible process of soul evolution with rebirth into ascending grades of form and consciousness as its machinery, The first by itself would mean only a cosmic evolution; for the individual would be a quickly perishing instrument, and the race, a more abiding collective formulation, would be the real step in the progressive manifestation of the cosmic Inhabitant, the universal Spirit: rebirth is an indispensable condition for any long duration and evolution of the individual being in the earth-existence, Each grade of cosmic manifestation, each type of form that can house the indwelling Spirit, is turned by rebirth into a means for the individual soul, the psychic entity, to manifest more and more of its concealed consciousness; each life becomes a step in a victory over Matter by a greater progression of consciousness in it which shall make eventually Matter itself a means for the full manifestation of the Spirit, "

Sri Aurobindo
The Life Divine

   It is difficult to understand, Sweet Mother.
   Ah!...
      If you take terrestrial history. all the forms of life have appeared one after another in a general plan, a general programme. with the addition, always of anew perfection and a greater consciousness. Take just animal forms -for that is easier to understand, they are the last before man -each animal that appeared had an additional perfection in its general nature -I don't mean in all the details -a greater perfection than the preceding ones, and the crowning point of the ascending march was the human form which, for the moment, from the point of view of consciousness, is the form most capable of manifesting consciousness; that is, the human form at its height, at the height of its possibilities, is capable of more consciousness than all preceding animal forms.
  This is one of Nature's ways of evolution.
   Sri Aurobindo told us last week that this Nature was following an ascending progression in order to manifest more and more the divine consciousness contained in all forms. So, with each new form that it produces, Nature makes a form capable of expressing more completely the spirit which this form contains. But if it were like this, a form comes, develops, reaches its highest point and is followed by another form; the others do not disappear , but the individual does not progress. The individual dog or monkey, for instance, belongs to a species which has its own peculiar characteristics; when the monkey or the man arrives at the height of its possibilities, that is, when a human individual becomes the best type of humanity, it will be finished; the individual will not be able to progress any farther. He belongs to the human species, he will continue to belong to it. So, from the point of view of terrestrial history there is a progress, for each species represents a progress compared with the preceding species; but from the point of view of the individual, there is no progress: he is born, he follows his development, dies and disappears. Therefore, to ensure the progress of the individual, it was necessary to find another means; this one was not adequate. But within the individual, contained in each form, there is an organisation of consciousness which is closer to and more directly under the influence
        of the inner divine Presence, and the form which is under this influence -this kind of inner concentration of energy -has a life independent of the physical form -this is what we generally call the "soul" or the "psychic being" -and since it is organised around the divine centre it partakes of the divine nature which is immortal, eternal. The outer body falls away, and this remains throughout every experience that it has in each life, and there is a progress from life to life, and it is the progress of the same individual. And this movement complements the other, in the sense that instead of a species which progresses relative to other species, it is an individual who passes through  all the stages of progress of these species and can continue  to progress even when the species have reached the limit of their possibilities and. ..stay there or disappear -it depends on the case -but they cannot go any farther, whereas the individual, having a life independent of the it purely material form, can pass from one form to another and continue his progress indefinitely. That makes a double movement which completes itself. And that is why each individual has the possibility of reaching the utmost realisation, independent of the form to which he momentarily belongs.

The Mother