interests. The field for the legitimate exercise of the police power is coextensive with the changing needs of society. The record does not negative the existence of any of the possible findings that have been mentioned. All rational presumptions are in favor of the validity of an act of the legislative department of the government. Perkins v. Westwood, 226 Mass. 268. Moore v. Election Commissioners of Cambridge, 309 Mass. 303. Morrissey v. State Ballot Law Commission, 312 Mass. 121, 130.
It is alleged in the bill that the plaintiff customarily sells only one grade of gasoline, and that it is more necessary for it to depend on signs advertising the prices than some of its competitors. The allegations of the bill, however, are not sufficient, either in terms or by necessary implication, to show that the plaintiff sells but one grade of gasoline, and no such conclusion is admitted by the demurrer. Johnson v. East Boston Savings Bank, 290 Mass. 441, 446, 447. Comerford v. Meier, 302 Mass. 398, 402. We do not intimate that if the plaintiff was selling only one grade of gasoline, or even if it was the only dealer in this Commonwealth selling a single grade, it would be immune, on constitutional grounds, from the reach of the statute. Massachusetts General Hospital v. Belmont, 233 Mass. 190. Milton v. Donnelly, 306 Mass. 451. Wickard v. Filburn, 317 U. S. 111, 129, 130.
One assailing a statute on constitutional grounds has the burden of proving the absence of any conceivable grounds upon which the statute may be supported. "As underlying questions of fact may condition the constitutionality of legislation of this character, the presumption of constitutionality must prevail in the absence of some factual foundation of record for overthrowing the statute.” O’Gorman & Young, Inc. v. Hartford Fire Ins. Co. 282 U. S. 251, 257-258. ."The burden is not sustained by making allegations which are merely the general conclusions of law or fact. . . . Facts relied upon to rebut the presumption of constitutionality must be specifically set forth.” Pacific States Box & Basket Co. v. White, 296 U. S. 176, 185. Congress in dealing with filled milk products, U. S. C. (1940 ed.) Title