NOTE that this is a review of the Harvard University Press facsimile of the first edition of "On the Origin of Species" (intro by Ernst Mayr). This is NOT a commentary on Darwin's text.
I blithely bought and began reading the Modern Library's "Origin", then came across this facsimile of the first edition in the library. Hmm, I wondered. I used the quotations in the front of my copy to deduce that I was reading the sixth (and last) edition, rather than the first. While that, too, has its considerable interest in illustrating the twists and turns of Darwin's thought during those years, the evolution revolution was made by the first edition. As Ernst Mayr says in his introduction, "When we go back to the Origin, we want the version that stirred up the Western world, the first edition." Besides which, if one is going to do any historical research, one needs this edition, for contemporary references use the first edition's pagination.
But most importantly, this is the firstborn of Darwin's mind, long gestating, and contains his most confident and positive statement of his thesis. He had tried to anticipate all the major objections to his theory and answer them preemptively here. Still, at the time of this writing he had no critics, so the tone and content display none of that waffling that mar, to a certain extent, the final edition.
This volume was put together in 1964, and Ernst Mayr's introduction dates from that time. It is a good historical introduction to Darwin and his contribution, and some more specific remarks on the first edition, its general approach and some of its path-breaking arguments. Also included in the extra matter is a bibliography of Darwin's published works, plus current works on evolution, as of 1964. There is also a quite comprehensive index of the text, which should make the book considerably more usable to us than it was to Darwin's original readers.
My only gripe is that Harvard University Press only offers a paperback, although it used to have a hardcover edition. The paperback version is readable enough at 5.5 by 8.2 inches, yet it's too thick for its size, and, while definitely not of poor quality, vulnerable to the binding breakage typical of the breed, so serious scholars of the work might find themselves literally pulling it apart. For you and me, though, it should be just fine.