Appendix. 



not completed during the epoch under consideration, but during 

 it, the presumption was clearly established that on fuller investi- 

 gation a solution would be found of all cases that had hitherto 

 baffled detection. The latest of these investigations worthy of 

 note are those of Leuckart (1856-7) and Virchow (1858) upon 

 the trichina spiralis ; and they have confirmed in a very positive 

 manner the opinion just stated, for it is a reasonable and almost 

 universally admitted canon in scientific study that it is. more 

 probable that a law which is known to be without exception in 

 phenomena, which we can clearly trace, extends to similar phe- 

 nomena not yet fully explained, rather than that a new law 

 should now come into play. This, of course, does not exclude 

 the possibility of a new law, and the true scientist will cheerfully 

 accept such a law, whenever by observation, comparison and ex- 

 periment its correctness is established. 



2d Epoch (continued}. The other phase of the epoch under 

 consideration relates solely to the origin of infusorial life. The 

 microscope had been of great service in enabling scientists to 

 account for the mode of generation in known animals, but with 

 all this extension of knowledge it had also brought into view a 

 new outlying territory which swarmed with animal life in num- 

 bers and kind before unsuspected. These are the infusoria first 

 discovered by Leeuwenhoek in 1675 an< ^ called by him anima- 



. 

 migration of parasites a subject just then being, established he took with 



him from Louvain to Paris four pups which he had reared. Two of them he 

 had fed upon the cysticercus cellulosus of the rabbit, the larval form of the 

 tcenia serrata of the dog. These pups he presented to a commission 

 of scientists (Valenciennes, Milne Edwards, Quatrefages and Jules Haime) 

 saying : In two of these dogs you will find not a single specimen of 

 tcenia serrata, in the other two you will find many; and furthermore, in this 

 one you will find specimens in four different stages of development, while in 

 that one you will find them only in three stages, and the number of speci- 

 mens in this dog is much greater than in that one. The pups were then killed 

 and his statements were proved to be absolutely correct. In one dog, how- 

 ever, some tceniae cucumerinae were found, and Van Beneden frankly owned 

 lie could not tell where they came from. Since then it has been discovered 

 that they originate from an acarus, the trichodcctes, which lives in the hair of 

 dogs and which is infested by the scolex of this variety of tape- worm. The 

 dog licking its hair swallows the acarus and thus infects itself very much in 

 the manner in which a horse is infected with bots, by licking up the eggs of 

 the oestrus or gad fly. 



