ORIGIN AND TRANSFORMATION OF ANIMALS. 671 
other is built up.” The moults of crabs and other Crustacea 
do not prove exceptions to this rule, for although the actual 
change of the hard integument appears sudden, the internal 
processes which lead to it have been gradually carried on. 
Among insects the larva prepares 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 unex¬ 
pected facts. “ He found that all through the fine weather 
the aphides reproduce their race, if isolated, but when the 
temperature falls, these animals, returning to ordinary con¬ 
ditions, 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 
position 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 Cha- 
misso should make his discovery that the Salpse produce 
their offspring in the strange fashion which he characterised 
as the “alternation of generations;” and here we cannot do 
better than borrow M. Quatrefages* description of a salpa, 
for the benefit of those to whom this interesting inhabitant 
of our seas is unknown. He says: “ Salpae 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 repre¬ 
sents the mantle and the shell of ordinary molluscs, and it 
is pierced towards each extremity. The water necessary for 
its respiration enters at one of these apertures and is ex¬ 
pelled 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 
