Life essentially utilizes unorganized or lesser organized matter and pulls it together into an organized form which is the life form. Life forms have maintained their survival through evolution. The process by which environmental constraints act in a selective manner to differentially favor variations in offspring that confer greater benefit to one's evolutionary fitness. Evolutionary fitness is a measurment of an individual's ability to contribute to the successful spreading of his genes in future generations. Successfully spread genes are able to successfully spread again and again.
Selection is based on environmental critera.
Evolution is generally thought of in terms of progress, since environmental constraints to evolutionary fitness often appear to become more and more complex. As a result, evolution often progresses towards increasing complexity. However, the idea of progress does not exist, it is merely an illusion. The struggle for existence never gets easier. However well a species may adapt to its environment, the challenge of adaptation is never met.