12 INTRODUCTION. 



certain degree of development elsewhere. This is particularly 

 the case with such intestinal worms as remain parasitic in the 

 last stage of their existence, viz. that of sexual maturity, whilst 

 the Gordiacai (Gar dim and Mermis), as soon as they are full- 

 grown, quit their parasitical life, in order to become sexually 

 mature, away from the animal they have infested. During these 

 early wanderings, the worms in question commonly undergo a 

 change of form -a sort of metamorphosis, often accompanied by 

 other phenomena of so highly remarkable and abnormal a 

 character, that naturalists could not at first understand the varied 

 character and import of these phases of existence, nor comprehend 

 their relation with hitherto known facts. 1 For a long time it 

 was supposed that these discoveries were isolated facts, and they 

 were regarded as a sort of curiosity; but here again the saying 

 was verified, that that which at first appeared to be the exception, 

 eventually proves to be the rule. By degrees, a mass of obser- 

 vations upon certain remarkable metamorphoses of the intestinal 

 worms accumulated, and constituted a complete chaos of seem- 

 ingly irregular phenomena, which broke down every barrier 

 hitherto set by the acknowledged laws of animal existence 

 and propagation, until the penetration of the Danish naturalist, 

 Steenstrup, 2 succeeded in evolving a certain order out of this 

 confusion, by the discovery therein of a hidden, underlying law 

 of nature, by which all the phenomena that had seemed so devoid 

 of plan could be reduced to order. Steenstrup named the newly 

 discovered law, the " Alternation of Generations," a phrase which 

 describes this phenomenon. " That an animal bears young which 



1 I may refer to the " king's-yellow" worms discovered by Bojanus in water-snails, 

 and now become famous. (See Oken's ' Isis,' 1818, p. 729, plate 9, figs. A, F.) Of this 

 discovery Oken says 4< Observations of this kind make one dizzy." No less attention 

 was excited by Von Bar's description of the Bucephalus pclymorphus of the fresh-water 

 mussel. (See ' Verhandlungen der Kaiserl. Akad. d. Naturforscher,' B. xiii, 1826, 

 p. 570, pi. 30) ; and by the Leucochloridum paradoxum, first discovered by Ahrens, and 

 afterwards described anew by Carus. (See ' Magazin der Naturforschenden Freunde zu 

 Berlin, 1810, p. 292, pi. 10, figs. 12 19, and the ' Verhandlungen der Kaiserlichen 

 Akademie,' Bd. xvii, 1855, p. 87, pi. 7.) 



2 See his important essay on the ' Alternation of Generations,' Copenhagen, 1842. 

 [This essay has been translated by Mr. Busk, and forms one of the publications of the 

 Ray Society. It must not be forgotten that the first conception of the doctrine of the 

 " Alternation of Generations," and the first use of the term, are due to Chamisso. See his 

 ' De Anirnalibus quibusdam e classe vermium Linnaeana,' 1819, and ' Reise urn die 

 Erde.' ED.] 



