Byron White, Lewis Publishing Company v. Morgan…
“ We come, then, to determine whether the provision as thus construed is valid. That Congress, in exerting its power concerning the mails, has the comprehensive right to classify which it has exerted from the beginning, and therefore may exercise its discretion for the purpose of furthering the public welfare as it understands it, we think it too clear for anything but statement; the exertion of the power, of course, at all times and under all conditions, being subject to the express or necessarily implied limitations of the Constitution. ”
