Interesting you pointing to jack London work . I only read one of his book long time ago
"Sea -Wolf " and enjoyed it. He was follower of Darwin , Spencer , Marx and Nietzche.
Initially he was socialist but few months before his death he resigned from socialist
party. I kept follwing excerpts from his book "The call of the Wild" on my favorites.
"There is an ecstasy that marks the summit of life, and beyond which life cannot rise. And such is the paradox of living, this ecstasy comes when one is most alive, and it comes as a complete forgetfulness that one is alive. This ecstasy, this forgetfulness of living, comes to the artist, caught up and out of himself in a sheet of flame; it comes to the soldier, war-mad on a stricken field and refusing quarter; and it came to Buck leading the pack, sounding the old wolf-cry, straining after the food that was alive and that fled swiftly before him through the moonlight." (from The Call of the Wild)