In addition to the wonderful Darwin's Writings on the Web site, there's also a Lamarck on the Web, an Alfred Russel Wallace site, and a Buffon site. Put your favourite historical biology sites in the comments, and I'll assemble a general reference page.
you may browse through Electronic Scholary Publishing http://www.esp.org/
you may also visit http://www.biologie.uni-hamburg.de/b-online/d00/klassik.htm which includes Mendel's famous article and some stuff of Haeckel
A collection of historic and modern biology books:
http://www.biolib.de/
132135 scanned pages out of 430 books
Don't forget this site:
http://mdot.nlm.nih.gov/proj/ttp/books.htm
with Conrad Gesner?s Historiae Animalium and Andreas Vesalius's De Humani Corporis Fabrica.
I'm sure you have some of these already:
The Longest Running Falsehood in Creationism
On the Vegetation of the Galapagos Archipelago
For the terminally desparate, there's the RA Fisher digital archive.
Bob