These three works give us all the philological discoveries made during the last thirty years in the French language; and the chasm which separates them from the dreams of Menage and Roquefort can only be compared to that which lies between the chemistry of Lavoisier and the reveries of Raymond Lulli, or Van Helmont. It may there fore seem needless to wish to swell the catalogue with a new philo logical Dictionary; but still I have decided on writing this book; for there is a blank to be filled up. In scientific subjects there is always room for two kinds of books — those which teach established scientific knowledge and transmit our learned acquisitions in a collective form, and those which leave former discoveries alone, in order to at tempt new research, to work out the solution or the discussion of problems hitherto untouched. Thus, in zoology, a treatise intended for the general public would be silent as to all doubtful or unsettled questions (such as the origin of species, or the like), and would occupy itself solely with the minute proof of established truths: but if on the other hand the treatise were addressed to the narrower class of professed naturalists, it would be satisfied with simply stating known facts (assuming their proof to be known by the reader) and would set itself specially to elucidate by new observations or hypotheses those problems which were yet uncertain.