Only Mackenzie is credited with authorship on the title page. Holland and Bright accompanied Mackenzie on this voyage and exploration. On p. xi of the Preface Mackenzie indicates that he benefitted from Hooker's notes on the botany of Iceland. On p. xiii he credits Holland with the account of the diseases of the Icelanders, and credits Bright for the account of the zoology and botany of Iceland. Digital facsimile from Google Books at this link.