It's called "The Origin of Species" not "The Origin of Variations Within Pre-Existing Species".
Fail.
Main Entry: species
Part of Speech: noun
Definition: class, variety
Synonyms: breed, category, collection, description, division, group, kind, likes, lot, nature, number, order, sort, stripe, type
Notes: species is a subgroup of genus and you capitalize a genus, but not a species (which is properly pronounced SPEE-sheez)
http://thesaurus.reference.com/browse/speciesCharles Darwin's book was titled
The Origin of Species because it was intended to explain the existence of species, which can be defined as variety. Had Darwin intended for his book to explain the origin of life itself he would have titled it
The Origin of Life.