Nitpick:
The Secret of Life was infinitely beyond the reach of science! Not just a little beyond, mind you, but infinitely beyond!
But Kelvin (in your quote) qualified it with "... hitherto entered on". Whether or not "infinitely" is fitting, doesn't this imply that Kelvin did not think that future scientific inquiry could not succeed?
What concept in the Babyeaters' language is the humans' "good" translated to? We have been given a concrete concept for their terminal value, but what is theirs for ours, if any?