Thus we find some which are really at home, and others which are on their journey, sometimes on the right road, and at others, wandering and lost in an alien “host.” The former are autochthonic parasites, the others are foreigners. We may say that each animal species has its proper parasites, which can live only in animals which have at least more or less affinity with
their peculiar host. Thus the Ascaris mystax, the guest of the domestic cat, lives in different species of Felis, while the fox, so nearly resembling in appearance the wolf and the dog, never entertains the Tænia serrata, so common in the latter animal.
The same host does not always harbour the same worms in the different regions of the globe which it inhabits. This relates both to the parasites of man, and to those of the domestic animals. Thus the large tapeworm of man, which naturalists call Bothriocephalus, is found only in Russia, Poland, and Switzerland. A small tape-worm, Tænia nana, is observed nowhere except in Abyssinia; the Anchylostoma is known at present only in the south of Europe and the north of Africa; the Filaria of Medina, in the west and the east of Africa; the Bilharzia, that terrible worm, has only been found in Egypt.
There are also parasitic insects dreaded by man, as the Chigoe (Pulex penetrans) which, happily, is only known in certain countries. Some, however, have become cosmopolitan, since man has introduced them wherever he has established himself.
The mammalia which live on vegetable diet have Tænia without any crown of hooks, and man, according to his teeth, ought only to nourish the Tænia mediocanellata. We find in a work on the Algerian Tænia, by Dr. Cauvet, that it is the Tænia inermis, that is to say, without hooks, which is the species common in Algeria. Among fourteen tæniæ which he had occasion to examine, there was not a single Tænia solium. I have said long since, that this species ought to be less widely spread than the tænia without hooks. The Tænia solium
comes from the cysticercus of the pig, the other from that of the ox; and Dr. Cauvet has ascertained that the latter, in the state of cysticercus, has already lost its crown.
We find extinct fossil genera and species in all the classes of the organic world. Is it the same with worms and animals of other classes which are only known in the condition of parasites? Had the Ichthyosauri and the Plesiosauri worms in their spiral cœcum like plagiostomous fishes, which resemble them so much in the digestive tube? We do not doubt this, and we should have been glad to give some demonstration of it. For this purpose, we have made a collection of the coprolites of these animals, but we have not yet succeeded in getting slices thin enough or sufficiently transparent to discover the eggs or the hooks of their cestode worms.
Not long ago, the partisans of spontaneous generation found in the class of worms their principal argument for their old hypothesis, and it was even after the publication of my treatise on intestinal worms that this question, which seemed forgotten, was taken up again by Pouchet. At present, they appear to have given up parasites, which reproduce their kind like other animals, and to have fallen back upon the infusoria, the last intrenchment which remained to the partisans of spontaneous generation, whence Mons. Pasteur has scientifically dislodged them. It is evident to all those who place facts above hypotheses and prejudices, that spontaneous generation, as well as the transformation of species, does not exist, at least, if we only consider the present epoch. We are leaving the domain of science if we take our arms from anterior epochs. We cannot accept anything as a fact, which is not capable of proof.