Download Charles Darwin's "Voyage of the Beagle" for free from Project Gutenberg. Choose the EPUB version and save it into iBooks. Do a search for either the scientific name or the common name of your animal using the iBooks magnifying glass in the top right corner, and it will direct you to the pages that reference said creature.
The University of Cambridge's Darwin Correspondence Project provides access to the transcripts and pictures of all of Darwin's correspondence. Search 8000+ letters and articles for your keword(s). The site also provides other relevant links and resources.