II. Darwin's Theory
A. All populations are genetically variable.  "New" variations can suddenly appear within a population.  These variations can be passed to the offspring.
B. (Thomas Malthus)  All populations will eventually reach a size where "more offspring are born, than can survive to breeding age."
C. A competition will occur between individuals for limited resources.
D. The environment selects those individuals best adapted to survive and pass on their genes. Natural Selection.