30 INTRODUCTION. 



their mode of origin, I will give an account of the results 

 below. 



With the migrations and alternation of generations amongst 

 the intestinal worms, two other.phenomena are connected, which 

 were formerly quite unnoticed, but which now, since attention 

 has been directed to them, have been very generally observed. In 

 the neighbourhood of those sexually perfect intestinal worms 

 which, in their wanderings, are subject to the alternation of 

 generations, only eggs, or recently hatched embryos are met with ; 

 but the further stages of development are always wanting, 

 since they first make their appearance after the emigration of 

 the young to other places. Further, many of these intestinal 

 worms, taken whilst in the act of migrating, are never found 

 below a certain size, since they do not commence their wanderings, 

 either as nurses or larvse, until they have already reached a certain 

 stage of their development. 



In this chapter I have expressed myself somewhat at large 

 upon the wanderings and alternation of generations of the intesti- 

 nal worms, in order that I may be fully understood in the 

 ensuing ones, when I have occasion to refer to this generation 

 by agamozooids. The history of the propagation of certain para- 

 sites, in the foregoing pages, may seem new and astonishing 

 to many readers, and yet the alternation of generations is not 

 more wonderful than metamorphosis. We have been so long 

 acquainted with the way in which metamorphosis takes place in 

 the higher and lower members of the animal kingdom, that we 

 no longer wonder at the various transformations of the frog, nor 

 gaze with surprise when a caterpillar becomes a chrysalis, and 

 after a certain time flies off in the shape of a butterfly. The 

 many to whom the metamorphosis of frogs and insects is a 

 common appearance, forget that there was once a time when it 

 was unknown, and when the multiplication of grubs and larvae 

 was ascribed to equivocal generation, their true origin being 

 unsuspected. It is to be hoped that a time will also arrive 

 when the complicated alternation of generations will not be 

 known to naturalists alone. 



