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.