Since the supposed differences thus are reduced to the one of number only, and since that turns out to be irrelevant and wholly without substance, it follows that the act is a 'clear and hostile discrimination' against a selected body of taxpayers, Bell's Gap R. Co. v. Pennsylvania, 134 U. S. 232, 237, 10 S. Ct. 533, 33 L. Ed. 892, a mere subterfuge by which the members of one group of taxpayers a