Sir J. Lubbock on the Metamorphoses of Insects. 379 
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 gen- 
erative 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 repro- 
duction will only result in those species which have the power of 
agamic multiplication. 
However this may be, insects offer every gradation between simple 
growth and that phenomenon which is known as alternation of gen- 
erations. 
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. 
In Chloéon, though the changes are gradual, the difference be- 
tween the larva and the imago is very considerable, and we have 
seen that the action of external forces produces changes which have 
no reference to the form of the perfect insect. 
In caterpillars we have a typical class of metamorphoses, 
Until recently, however, we knew of no case in which a larva pro- 
duced more than one perfect insect*. Insects never multiply by 
buds, and almost always the external form is acquired before the 
organs of reproduction are mature. Recently, however, Professor 
Wagner of Kasan has discovered that the larvee of certain Cecidomyias 
have the faculty of producing other larvee, so that they present a 
true case of alternation of generations. Thus, then, we see that 
insects present every gradation, from growth to alternation of gen- 
* The instances in which certain insects breed while their wings are but 
imperfect, might here have been cited. But as there is much difference of 
opinion among entomologists as to these cases, I have thought it better to 
take one about which no question is likely to arise. 
