Nothing is less to be desired than the fate of a young man who, as the Scotch proverb says, in “trying to make a spoon spoils a horn,” and becomes a mere hanger-on in literature or in science, when he might have been a useful and a valuable member of Society in other occupations.
I can surmise the meaning of it in that context. Huxley was convincing the young man to strike a balance between commercial work and academic pursuits. To avoid ruining his job while pursuing something he might not deserve.
I have searched the Google, only to find this: "make a spoon or spoil a horn", which is relative to but different from this phrase. Then my question is, what is the origin and the exact meaning of "trying to make a spoon spoils a horn"?