106 ANIMAL PARASITES AND MESSMATES. 



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 coecum like plagio- 

 stomous fishes, which resemhle 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 genera- 

 tion found in the class of worms their principal argu- 

 ment 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 

 sj)ontaneous 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. 



