Even as to commerce belonging to any of the three classes specified by the federal constitution, the power of state regulation is not necessarily denied. It is to be noticed that Congress is not expressly given the exclusive power to regulate, and while the power of Congress is necessarily superior to that of the states, so that any regulations which Congress may adopt will supersede state laws on the same subject, it does not follow that state legislation affecting the three forms of commerce enumerated and not in conflict with any laws of Congress on the same subject is necessarily invalid. Thus harbor regulations, or rules as to the employment of pilots, may affect foreign or interstate commerce, but they are not on that account ineffectual if there are no statutes of Congress with which they are in conflict. Again, the erection of bridges over navigable streams within state limits, or the construction of dams in such streams, can be authorized by the state, although foreign or interstate commerce is thereby, to some extent, interrupted. There are many local regulations essential for the control of commerce under peculiar circumstances, which cannot very well be provided by general law, and so far as the state regulations do not interfere with the general law on the subject they are regarded as proper.

The fact that state regulations adopted in the exercise of the general police power may incidentally affect foreign and interstate commerce does not render such state regulations necessarily invalid. If they are not unreasonable, nor calculated to effect a discrimination, and do not in substance amount to general regulations of such commerce as is placed within the control of Congress, they will be upheld. For instance, a state may require locomotive engineers to procure a license, although they are to act for railroads engaged in interstate as well as internal commerce. In the absence of any legislation by Congress, this principle would also apply to engineers on vessels navigating the public waters within the state, and also to the inspection of such vessels, but if there is congressional legislation on this subject, any state legislation is thereby superseded. The states can also, without doubt, prohibit the sale of goods dangerous to the public health or morals, even though brought from another state or from a foreign country. As the general police power is left to the states and is not vested in Congress, it is evidently necessary that the states shall exercise it, not only with reference to goods produced in the state, but also as to goods brought into the state from without.