Once we have reached the human level we do not regress back into animal forms. The law of karma acts over our successive lives to ensure justice. We are the cause of every joy and pain in our own life. A law of cycles provides fundamental structure at all levels. Analogy and correspondence provide fundamental structure for the universe.
Evolution applies on a grand scale to all of life.
Soul gets involved in this world of matter. It experiences and learns. Soul then works its way back in a long pilgrimage to its primal source. This again follows a cyclic pattern. This evolution achieves experience, self-awareness, and ever increasing perfection.