100 The Origin and Transformation of Animals. 



ment appears sudden, the internal processes which lead to it 

 have been gradually carried on. Among insects the larva pre- 

 pares the materials which the chrysalis will require ; it has, so 

 to speak, stored up in a magazine the materials necessary for 

 its transformation. 



We now come to the class of facts which M. Quatrefages 

 groups together under one term, " Geneagenesis." As our 

 readers will probably know, those vexatious inhabitants of the 

 greenhouse or garden, the plant-lice, or Aphides, produce a 

 series of offspring without the conjunction of two sexes, and in 

 this mode of proceeding, Bonnet discovered unexpected facts. 

 e ' He found that all through the fine weather the aphides repro- 

 duce their race, if isolated, but when the temperature falls, these 

 animals, returning to ordinary conditions, propagate by eggs 

 which demand the conjoint action of a father and a mother. 

 These eggs pass the winter glued to the branches of the trees on 

 which the colony dwelt that was destroyed by the cold. When 

 they are hatched in spring they yield viviparous individuals only; 

 in the autumn males and females appear, and from this moment 

 oviparous generation recommences its work." It would have 

 been impossible to place these curious incidents in their true posi- 

 tion if Trembley and others had not observed that polyps and 

 similar animals of simple structure can propagate like vegetables 

 by buds. It was also necessary that Chamisso should make his 

 discovery that the Salpge produce their offspring in the strange 

 fashion which he characterized as the "■ alternation of genera- 

 tions ;" and here we cannot do better than borrow M. Quatre- 

 fages' description of a salpa, for the benefit of those to whom 

 this interesting inhabitant of our seas is unknown. He says : 

 " Salpse are marine molluscs of a very queer shape, which it is 

 difficult to describe. We may, however, figure one as an 

 irregular crystal cylinder, perfectly transparent, in the interior 

 of which is suspended a proportionably small mass of opaque 

 lively coloured matter, called the nucleus. This is formed by 

 the junction of the principal viscera. The cylinder represents 

 the mantle and the shell of ordinary molluscs, and it is pierced 

 towards each extremity. The water necessary for its respira- 

 tion enters at one of these apertures and is expelled from the 

 other, thanks to the contractions of the mantle ; and making its 

 exit with rapidity, it pushes the animal in an opposite direction, 

 so that the creature swims solely by means of its respiratory 

 movements.-" For a long while the attention of naturalists was 

 drawn to these objects, " whose phosphorescence was remark- 

 able even among the fiery waves of the intertropical ocean," 

 and they sometimes discovered them in an isolated condition, 

 and sometimes in chains. It was Chamisso who explained this 

 riddle. He saw that the' Salpse were androgynous (bisexual) and 



