"----I have called this principle, by which each slight variation(a), if useful, is preserved, by the term of natural selection-----". (emphasis added)
Thus, selection by nature of one of a (varied) species (for survival) is the explanation for evolution of that species. This statement is equivalent to the conclusion reached by using the more popular term "survival of the fittest". This latter description of the principle of evolution must be considered a more straightforward term, and it is not hard to understand why the term was not by used originally by Darwin. Only two questions must be asked about the term "survival of the fittest" to uncover it's true meaning. One question is "what are the "fittest" and the other, how are the "fittest" chosen?"
The answer is much too simple and also much too revealing: the fittest are those who survive (Period). Why are they the fittest? Because they survived. But why did they survive? Because they are the fittest. That's all we know about them. The same answer applies to the more obscure phrase, (survival or "preservation") by "natural selection": the types that have been selected by nature are those who have survived. Both arguments are tautological. Nothing is learned about the real scientific cause of evolution or the origin of species by learning that the population consists of survivors. It also explains nothing that we don't already know about the world around us. Yes, elephants have "survived". Yes, so have fleas, oak trees, bacteria, etc.. In Darwin's terms, these species have been selected. Quite obvious. They are here. What is the cause of their differences? It is certainly not the fact that they have survived.