Underinclusive legislation

The legislature is not bound, in order to support the constitutional validity of its regulation, to extend it to all cases which it might possibly reach. Dealing with practical exigencies, the legislature may be guided by experience. It is free to recognize degrees of harm, and it may confine its restrictions to those classes of cases where the need is deemed to be clearest. As has been said, it may 'proceed cautiously, step by step,' and 'if an evil is specially experienced in a particular branch of business' it is not necessary that the prohibition 'should be couched in all-embracing terms.' If the law presumably hits the evil where it is most felt, it is not to be overthrown because there are other instances to which it might have been applied.