GERMAN VERSION. 5 



the sexual apparatus ; so that the alternation of genera- 

 tions in this point of view becomes in its nature a neces- 

 sary or unavoidable step of the development, and we 

 may in the words of the great master, affirm, that " Na- 

 ture pm'sues its com-se, and what we take for an exception 

 is in accordance with law." 



In what precedes, the object of the Essay, and the 

 modes in which the results at which I beheve myself to 

 have arrived, have been obtained, will be sufficiently ap- 

 parent. The " alternation of generations" is, as has been 

 said, the fundamental idea followed throughout the book; 

 1 have wished to represent it in the reality which I am 

 convinced it possesses, from the researches of others as 

 well as from my own, however much it may have been 

 combated, in the one particular in which an ingenious 

 naturalist surmised its existence, even twenty years ago ; 

 I have also been desirous of reviewing this phenomenon 

 and its real significance, so far as I have as yet observed 

 them in natm-e. With regard, however, to other points 

 which have come out during the course of these ob- 

 servations and researches, viz., that the Cercarim are larva 

 of entozoa, of the genus Distoma, and in fact of those 

 species which inhabit the interior of fresh water snails 

 (in the liver, &c.) ; that the entozoa pass part of their 

 existence in a state of freedom in the water external to 

 the snail, which they afterwards inhabit, and that they 

 re-enter them from without;* that whole established 

 divisions of families of animals must be abolished, since 

 they include only undeveloped forms, or forms which 

 bear the same relation to the true and perfect form of the 

 species, that the " workers" among ants and bees bear 

 to the fertile females of those insects ; f and, finally, that 

 several forms which have been considered as of different 



* Facts wliich I can now multiply from several other species besides those 

 wliicli I have adduced. 



t This may doubtless be assumed of most, if not of all the genera of the 

 Co ryne family, and of Siebold's division of the "Asexual Trematoda" &c. 



