INTRODUCTION. XXlll 



nounced to the " Institut de France," that cestode worms 

 must necessarily pass from one animal to another in 

 order to complete the phases of then' evolution. 



At the present time, experiments respecting these 

 transmigrations are repeated every day in the labora- 

 tories of zoology with the same success ; and Mons. E. 

 Leuckart, who directs with so much talent the Institute 

 of Leipzig, has discovered, in concert with his pupil 

 Mecznikow, transmigrations of worms accompanied by 

 changes of sex ; that is to say, they have seen nematodes, 

 the parasites of the lungs of the frog, always female or 

 hermaphrodite, produce individuals of the two sexes 

 which do not resemble their mother, and whose habitual 

 abode is not in the lungs of the frog but in damjD earth. 

 In other words, let us imagine a mother, born a widow, 

 who cannot exist without the assistance of others, pro- 

 ducing boys and girls able to provide for themselves. 

 The mother is parasitical and viviparous, her daughters 

 are, during their whole life, free and oviparous. 



This observation leads us to another sexual singu- 

 larity, lately observed, of males and females of different 

 kinds in one and the same species, and which give birth 

 to progeny which do not resemble each other; the same 

 animals, or rather the same species, proceed from two 

 different eggs fecundated by different spermatozoids. 



Now that these transmigrations are perfectly known 

 and admitted, the starting-point of the inquiry has been 

 so entirely forgotten that the honour of the discovery 

 has been frequently attributed to fellow-workers, who 

 had no knowledge of it till the demonstration had been 

 completed, and the new interpretation generally accepted. 

 But let us retm-n to our subject. 



