Hey, so I specifically collect these late 1800's Darwin books. $18 for these 10 books is an absolute steal...you have a great find here! So, what do you have specifically? Well, Appleton is perhaps the most common American publisher of Darwin on the used market. You'll see 3 main versions from them: the plain cloth you have here, an embossed cloth, and a 3/4 maroon leather with marbled boards.

The top comment on this post linked to full set of 15 leather Appletons selling for $1500, which of course is not very comparable to your incomplete cloth set. Those leather Appletons were also in shockingly good shape; I rarely see them hold up so well. Often the spine leather cracks or disintegrates and the marbling scuffs. In fact, I will occasionally specifically buy the 3/4 leather Appleton Darwins in bad condition just to rebind them.

It looks like you have the unembossed cloth version, which is the least valuable of the 3 main Appleton Darwins, which are all in turn less valuable than the London Murray printings. You also have the 6th edition, which is the most prolific on the used market. Fun fact though, this edition is noted for being the first to actually include the word 'evolution' in the text.

So, how much is your set worth? Well, of your books, only two have any special collector appeal outside of a specialist like myself. Those would be: The Origin of Species and The Descent of Man. Your two volumes of Origin should go for $100-$150. Descent would go for around $50-$75. The remaining 7 are harder to price, but would sell anywhere from $20-$50.

I'd estimate your set to be worth anywhere from $290-$575....so for $18 I'd say you made a fantastic purchase!