Following McCalman's lead, I have gathered portraits (in the public domain) of each of these voyagers because one needs to keep in mind that they were young men. Each shows the man shortly before or after his journey of discovery.
The portrait below of Charles Darwin was painted in the 1830s by George Richmond. Darwin would have been in his very late 20s or early 30s and just returned. (This has been downloaded from Wikimedia Common which states it is in the public domain.)
Joseph Hooker is seen below at age 32 in an image from Leonard Huxley's Life and Letters of Sir Joseph Dalton Hooker (Volume 1, p. 340, 1918). This was some six years after the voyage that Calman describes in his book.
Huxley is pictured below in a daguerreotype taken in 1846 when he was 21. It is the frontispiece of Volume 1 of The Life and Letters of Thomas Henry Huxley, by Leonard Huxley (1901). Here he is about to begin his voyage.
Finally, this is Wallace in 1848 when he was 25, just as his voyages were about to begin. It is taken from his My Life: A Record of Events and Opinions (Volume 1, 1905, p. 264).
It is only appropriate to include an image of d'Orbigny. Clearly, he is young in this image, but I cannot read the script that appears below his portrait that might help me date it. This is from the frontispiece to a work that d'Orbigny edited, titled Voyage Pittoresque Dans Les Deux Amériques (1841). (Google Translate renders this title in English as Picturesque Voyage In The Two Americas.)