Immutable species fixity allows for slight variation between individuals but it does not allow for new species arising or acquiring new characteristics in daughter generations. Fixism is an idea that species in the past appeared as they do today. Species are immutable, forever stable no matter how many generations pass.
This is not a theory as it offers no explanation or mechanism to explain all observed data better than the theory of descent with modification. Biologists accept and use the modern evolution synthesis theory as a key element to understand all aspects of biology and the observed changes in species over time. With the addition of genetics and population biology Evolution is defined as changes in the frequencies of alleles within a population. Today most theories in paleontology, devlopmental embryology, comparative anatomy, taxonomy, geographic distribution, and even in molecular biology are subtheories of the prototypical descent theory.
National Center for cience Education - http://ncse.com/cej/8/1/theory-fact-evolution