of the old skin, by which the alterations which have perhaps been in 
preparation for days or even weeks, are rendered visible. In fact, there 
can be no great change in insects without a moult. Insects have no 
bones, and the muscles are attached to the skin, which therefore is 
necessarily hardened to afford them a solid and sufficient fulcrum. 
But it follows from this that no change of form can take place without 
a change of skin. 
In Chloeon we have seen that each moult is accompanied by a slight 
change. 
In caterpillars, on the contrary, there is little alteration during 
growth, and the changes are concentrated, so to say, on the last two 
moults. The advantage of this is obvious; the mouth, digestive, 
and other organs of the larva are very different from those of the perfect 
insect ; and if the change from the one type to the other were gradual 
and slow, the insect would be liable to perish of starvation in the midst 
of plenty. 
3. Similar considerations throw much light on the immobility of 
the pupa. The organs are altering so rapidly that they are unable to 
perform their functions. When the changes are gradual, as in 
Orthoptera, &c., there is no period of quiescence. 
The speaker then pointed out the analogy between metamorphoses 
and the alternation of generations. 
Many species of the lower animals are represented by two totally 
dissimilar forms ; but, so far as the speaker knew, no explanation of 
this remarkable phenomenon had yet been given. 
Through the metamorphoses of insects, however, we get a clue. 
When an animal is born in a state so early, that external forces act on 
it in one way, and on the perfect form in another, they tend to produce 
greater and greater differences between the two. As long as the 
external organs arrive at their mature form before the generative 
organs are fully developed, we have cases of metamorphosis ; but if the 
reverse is the case, then alternation of generations is the result. 
The same considerations explain why in alternation of generations 
the reproduction is almost invariably agamic in the one form. This is 
because impregnation requires the perfection both of external and 
internal organs ; and if the phenomenon arises, as has just been sug- 
gested, from the fact that the internal organs arrive at maturity before 
the external ones, impregnation cannot take place, and reproduction 
will only result in those species which have the power of agamic multi- 
plication. 
However this may be, insects offer every gradation between simple 
growth and that phenomenon which is known as alternation of genera- 
tions. 
In the wingless Orthoptera, the young so closely resemble the 
perfect insects, that there is nothing which in ordinary language would 
be called even a metamorphosis. 
In those Orthoptera which eventually acquire wings, there is of course 
a well-marked difference. 
