344 REPRODUCTION. 



water to fix themselves, in their turn, to the body of another 

 mollusk, and hegin their transformations anew. 



524. But this is not the end of the series. The nurses of 

 the Cercaria are themselves the offspring of little 

 Fig. 365. worms of yet another kind. At certain seasons, 

 we find in the viscera of the Lymn&a worms some- 

 what like the nurses of the Cercaria in shape (fig. 

 365), but rather longer, more slender, and having a 

 much more elongated stomach (s). These worms 

 contain, in the hinder part of the body, little em- 

 bryos (a), which are the young nurses of figures 

 363, 364. This generation has received the name 

 of grand-nurses. 



525. Supposing these grand-nurses to be the 

 immediate offspring of the Distoma (fig. 362), as 

 is probable, we have thus a quadruple series of 

 generation. Four generations and one metamorphosis are re- 

 quired to evolve the perfect animal ; in other words, we find 

 no resemblance to the parent in any of its progeny, until we 

 arrive at the fourth generation or the great-grandson 



526. Among the Aphides, or plant-lice, the number of 

 generations is still greater. The first generation, which is 

 produced from eggs, soon undergoes metamorphoses, and then 

 gives birth to a second generation, which is followed by a 

 third, and so on ; so that it is sometimes the eighth or ninth 

 generation before the perfect animals appear as males and fe- 

 males, the sexes being then for the first time distinct, and the 

 males provided with wings'. The females lay eggs which are 

 hatched the following year, to repeat the same succession. 

 Each generation is an additional step towards the perfect state ; 

 and as each member of the succession is an incomplete ani- 

 mal, we cannot better explain their office, than by considering 

 them analogous to the larvae of the Cercaria, that is, as nurses.* 



* There is a certain analogy between the larvae of the plant-louse 

 (Aphis} and the neuters or working ants and bees. This analogy has 

 given rise to various speculations, and, among others, to the following 

 theory, which is not without interest. The end and aim of all alternate 

 generation, it is said, is to favour the development of the species in its 

 progress towards the perfect state. Among the plant-lice, as among all 

 the nurses, this end is accomplished by means of the body of the 

 nurse. Now a similar end is accomplished by the working ants and 



