The exposition of H. Spencer's evolutionary ethics would be attained only beginning with the late 1870s. Nonetheless the general lines of that project were already been anticipated by Spencer in a letter addressed to J.S. Mill, which gained large resonance after its publication by A. Bain in 1868. In this letter Spencer reaffirmed his adhesion to utilitarianism, but at the same time he criticized the mere "empirical" method of Bentham's and Benthamites' utilitarianism, pointing out the best way to "scientific" utilitarianism in the deduction from the "laws of life". The present article attempts to reconstruct the controversy which aroused between Spencer and Mill. The main subject of the dispute was Spencer's attempt to reconcile utilitarianism and intuitionism on the base of the evolutionary hypothesis shaped on the Lamarckian mechanism of the heredity of acquired characters. The reasons of Mill's rejection of Spencer's attempt would have become evident after the publication of the posthumous essay Nature, where Mill took up a clear position against an instinct-based ethics.