Hans,

There is nothing erroneous with "Evolution by means of Natural Selection" being described as a scientific theory. It fits the criteria of a theory in that it provides an explanation for observations of the physical world. This particular theory predicts that the frequency in alleles within a species will be determined by the conditions that species finds itself in ( a species is simply a group of organisms that can interbreed and produce fertile offspring).

Our "notion" of a species is certainly not impercise nor is it arbitrary. Evolution by means of Natural Selection is one of the most ground-breaking ideas of this or any other time. To discount it because it doesn't encompass the entirety of biologic study is surely an insult to the great scientists who contributed to this theory.