There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone circling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.
I sat up straight when I heard "are being evolved," a clear example of the progressive passive from a period where the construction was still innovative. Origin was published in 1859, when, as you can see from this Google Ngram, the progressive passive was still very rare:
The search phrases here (is being, are being, was being, were being) of course turn up some false positives, for example Samuel Johnson's quip, "being in a ship is being in a gaol, with the chance of being drowned." But most of the hits are real progressive passives, and you can Darwin's usage fits right on the early part of the upward curve.
Ten years later, Richard Grant White would unleash the full force of his venom against this construction, but from today's perspective, it seems hard to imagine it phrased any other way.
The construction that Jane Austin would have used at the start of the 19th century was the passival: "The clock struck ten while the trunks were carrying down" (Northanger Abbey).
If you try to recast Darwin's sentence in the passival, it becomes "are evolving," which would be ambiguous for voice: are the forms evolving or being evolved? It also becomes much worse writing:
"from so simple a beginning endless forms most beautiful and most wonderful have been evolved, and are evolving."
Only someone with a tin ear would think that this is better writing. 150 years later, of course, the innovation of the progressive passive is a dead issue, so of course it sounds better to us now, and White's complaints appear ridiculous to us. But there are still hundreds of equally churlish complaints still heard about people's writing with as little basis in common sense.
Here's a rule of thumb: if you have to preface a linguistic criticism by saying "I know it sounds totally natural, but..." you should probably shut up.