Bycatch (but not mine):
[left]: John Tenniel: Alice on the Train (1872)
[right]: Augustus Leopold Egg: The Travelling Companions (1862)
I found the comparison in preraphaelitesisterhood.com. If it is a pictorial reference at all, it might be a nice pun by Tenniel, but not as challenging as Henry Holiday’s conundrums.
Playing with the work of other artists could have been fun for John Tenniel too. (Of course another reason for such similarities always could be, that Holiday and Egg both referred an image by a third artist.)
2017-09-22, updated: 2021-05-24
But perhaps Holiday’s ruff – and the pose of the Fit Five drawing – was inspired by the Elizabethan drama inherent in Millais’ Boyhood of Raleigh, (1869).
Louise Schweitzer, One Wild Flower (2012)
If you want to be on the safe side, just claim that the meaning of the Snark is elusive. But to the more courageous readers I recommend Louise Schweitzer’s doctoral thesis One Wild Flower.
2017-09-04, update: 2021-03-05
Bycatch from my Snark hunt:
The, well, ambiguity of that “shadow”is known. Also there were some Freudian assumptions regarding what the salt could stand for. But so far I didn’t find any remarks on the impossibility of having a shadow being covered by white salt which isn’t covered by that shadow. To someone who learned physics that is a quite obvious question.
2017-12-17, update: 2020-04-11