A book published in 1859 was entitled, "Heterogenesis, or a Treatise on Spontaneous Generation." The author gives the clue to the origin of his errors in the second line of his preface, in which he says: "When, _by meditation_, it was evident to me that spontaneous generation was one of the means employed by matter for the reproduction of living beings."... According to this philosopher, science is, therefore, not the generalization of facts, but these facts must serve to prop up the theories or hypotheses invented in the silence of the study. This passage of his work shows us that he was no more able to yield to the evidence of experiments made on worms, than to those of Pasteur on the infusoria. It may be related to the honour of the illustrious Baer, that, from the year 1817, during his stay at Königsberg, he took up arms against this hypothesis, and never ceased to combat it, till evidence succeeded in opening the eyes of the most obstinate. The worms which present the most remarkable phenomena of transformations, accompanied by metamorphoses, are the Distomians and Cestodes, flat worms, which we will consider in the first place. Trematode worms include a certain number of large and beautiful parasites which scarcely undergo any change, and are found only on the skin and the gills of certain fishes; these are the monogenetic 189 trematodes, comprising the _Tristomidæ_ and all the worms of that group, which also stand higher in their organization: we shall speak of them hereafter. The other trematodes, which are called digenetic, live on the most dissimilar animals, under the most varied forms, and, like the greater part of the cestodes, introduce themselves into the individual who is to give them shelter, only by the assistance of a host, acting as a stage-coach which serves them as a vehicle. The principal family is that of the Distomidæ, a family _par excellence_ cosmopolitan; as inconstant in their progress as capricious in the choice of their companions. Each distome resembles a small leech which has a sucker in the centre of the belly, and as this sucker was once considered to be perforated, the name of Distoma was given to them. These parasites are the more interesting to us, from the fact that, though we are not the final resting-place of certain species, we nevertheless find them pass through us on their way. There are two species which occasionally lodge in the liver of man without being peculiar to him, for they properly belong to the sheep. Two other distomes have lately been described by Dr. Bilharz, which are fortunately only known at present in Cairo, and which are interesting, both with respect to their organization and to their manner of life.