The exact title (First Edition) was
On The Origin of Species or, The Preservation of Favoured Races in the Struggle for Life
The Sixth Edition re-titled the text
"The Origin of Species by means of Natural Selection or, The Preservation of Favoured Races in the Struggle for Life"
The text really centered on changes within 'kinds' (Darwins words not mine).
Of course Darwin had very limited knowlege, had he known what we know today he woudl probably have discared his entire notion as nonsense.. His original thesis was disproved before the close of the 19th century, modifictions made at that time to keep the thesis alive were disproved in 1953, the modern synthesis (Popularlized by Dawkin's The Selfish Gene) was disproved in 2010 and the model at this point has no working hypothesis as it awaits a fourth synthesis no doubt relying on gaps in our understanding of biology like all the previous models. The only modern theory that has been disproved more times than Darwins model is the Big Bang. Darwins model is an embodiment of the proverbial theory of the gaps, wherby it conveniently relies of areas of biolgy that are not fully understood and once on knowlege of those ares are expanded it must be reformulated to fit into new gaps in our understanding until the next gain in knowledge.. One can hope we we can make haste in filling in those gaps so we can finally discard the fairy story altogether.
In summary. The original theory relied on natural selection and transmission of acquired traits (Formerly know as Lamarkism).
Due to Gregor Mendels work the transmission of acquired traits was dropped and the assertion was made there was a continum of traits (genes) that allowed for unlimited variation.
The discovery of DNA in 1953 proved this continum of unlimited potential variation was false and the modern synthesis (Mutation + Selection) was adopted.
2010, Experiments in physiology proves most mutations are not random and those few that are generally act to knock out the gene forcing cells to use other pathways (since the mutated gene is effectively deactivated it is not subject to selection nor is there a reason the mutation should be preserved). A second discovery is that morphology is less a product of genes but rather a product of something in the cytoplasm of germ cells (dubbed epigenes)