Scientifically, evolution is defined as the process where more refined life forms grow and develop out of the less refined, that is to say, more advanced life forms arise out of lower forms; the gross producing the subtle. This is the theory of scientific evolution as propounded by Darwin. However, Tantra has a completely different concept of evolution. Tantra regards the refinement of life forms as being merely the pattern of the manifested world. It is the blueprint or design, but not the source of life as we know it. The true meaning of evolution revealed to the sages thousands of years before Darwin, is the process of Shakti arising or evolving out of Shiva, of the material evolving from the immaterial.
In this philosophy, life forms are not the result of the unfoldment of previous life forms, but every form of life has evolved from the underlying source of existence known as consciousness. The subtle does not evolve from the gross but the gross from the subtle. Consider a movie film. It has a story that unfolds from beginning to end. This is the process of scientific evolution. Each scene is related to the next but at the same time the story in the film is not the source of the pictures. The source of the picture is in the film strip. The film strip can be compared to consciousness. It is a projection of the film onto the screen from the film strip that Tantra and other Indian systems regard as evolution, not the flow of the story itself.
The tantric theory of evolution or manifestation of the material world takes place in two directions- the outward and the inward. In the outward direction, (pravritti). Shakti plunges downwards and product's the world of forms. In the inward direction (nivritti) Shakti moves in the opposite direction, from the gross to the more subtle. The further Shakti moves outwards and downwards, the deeper is the veil of ignorance. The greater the inward and upward movement of Shakti, the greater the awareness, knowledge or grace of divinity.
There is an exactly parallel concept in Taoism. Shakti, is known as 'Teh' which is divided into 'yin' (nivritti) and 'yang' (pravritti). Yin is the tendency to return to the source, to withdraw and submerge itself with the substratum of consciousness. Yang is expansion, action and creation of all things. They represent two rhythms - yang, or action and going forth, and yin, the return to the centre.
The aim of tantric practice is to reverse the downward and outward movement of Shakti and retrace one's steps along the inward and upward path to higher awareness. The goal is to reverse the direction of evolution and return to the source, or reunite Shakti with Shiva. Tantra divides people into two basic groups:
Most people are ideally suited to the pravritti path or marg. Tantra caters for both groups and prescribes specific practices for each. The path to enlightenment is first of all to follow pravritti marg to exhaust desires and fulfil all interest in the things of the world. In this manner one progresses naturally to nivritti marg. No force or unnatural effort is required. The process is entirely spontaneous. It is the nivritti marg that eventually leads to experience of the infinite, but most people are not ready for it, and must first enjoy life with all its facets to the full.
The practices of Tantra are designed, to lead the individual smoothly along both paths to the supreme experience. Tantra recommends those practices which are most suitable for the particular personality and one's stage of evolution. It realises clearly that there is no point in forcing the path of introspection, nivritti, on those who are not inclined towards it. Such is the depth and breadth of understanding of Tantra that it proceeds by natural means to lead all towards enlightenment without forming complexes which will create further barriers in the spiritual path and do more harm than good. Therein lies the greatness of Tantra.