There is another very specific quality which distinguishes them [man from animals], and which will admit of no dispute. This is the faculty of self-improvement, which, by the help of circumstances, gradually develops all the rest of our faculties, and is inherent in the species as in the individual: whereas a brute is, at the end of a few months, all he will ever be during his whole life, and his species, at the end of a thousand years, exactly what it was the first year of that thousand.