The difficulty of the problem we are here considering increases when we come to the inferior animals. Who would pretend to determine the number of species of _Mollusca_ which inhabit the earth, the fresh waters and the salt? This much is certain, that it cannot be less than that of the Vertebrates. * * * * * In the vast aggregate of the _Articulata_, the inquirer finds himself utterly astray and bewildered. This great division is not divided into those which have, and those which have not, articulated members. The first subdivision includes _Insects_, _Arachnida_, _Crustacea_, and _Myriapoda_; the second, _Annelida_ and _Entozoa_. Some naturalists, be it said, rank the _Cirrhopoda_ as intermediate between the two; others place them among the Mollusca. Others, again, include the _Rotifera_ in the second subdivision. We shall in this place confine our remarks to the _Insects_. According to the most distinguished entomologists, the average number of species at present, described or not described, and preserved in entomological collections, is between 150,000 and 170,000. This estimate is obviously below the truth. Take only the Coleoptera, which forms but one, though, it is true, the most numerous order of insects. Thirty years ago the most complete collections contained about 7000 species. In 1850, the museum at Berlin, according to Alexander von Humboldt, contained nearly 32,000. We would here call the reader's attention to the just remarks of the author of the "Natural History of the Coleoptera," an entomologist of great authority, whom a long residence in America had peculiarly qualified to pronounce an opinion on the subject before us:-- "If we remember," says the Count de Castelnau, "that there are immense regions in Asia and the two Americas of which we do not possess a single coleoptera; if we reflect that the interior of the vast continent of New Holland is, from this standpoint, entirely unknown, and that most of the archipelagoes of the great ocean have never been entomologically explored, we may conclude, without any fear of mistake, that the number of existing coleopteras exceeds _one hundred thousand_. However frightful this number may appear, it will seem less so if we examine only the species discovered in the neighbourhood of Paris, within a radius of twelve to fifteen leagues; and we do not hesitate to say, that in a few years the Parisian fauna alone will present material for a considerable work, which shall not treat of less than 3000 to 4000 species of Coleoptera."[84] * * * * *