The indispensable BHL is now even better: recently a ‘inside search’ button has been added, which allows the user to make a free search within the item under inspection. Read the full story here.
If you have used archive.org full text search you’ll know that the system shows the pages where your search term appears once you open the item.
The BHL interface doesn’t do that, and you have to repeat the search within the item, but when done it shows the contexts and gives page links.
Nevertheless, this will make searching for and within historical taxonomical literature much better and also save lots of time. Very nice!