6. That the form of the larva of each species depends in great 
measure on the conditions in which it lives. 
" When an animal is hatched from the egg in an immature form, 
the external forces acting upon it are different from those which affect 
the mature form, and thus changes are produced in the young, bearing 
reference to its present wants rather than to its ultimate form. 
" 7. When the external organs arrive at this final form before 
the organs of reproduction are matured, these changes are known as 
metamorpiioses ; when, on the contrary, the organs of reproduction are 
functionally perfect before the external organs, or when the creature 
has the power of budding, then the phenomenon is known as alter- 
nation of generations. 
" Insects present every gradation, from simple growth to alternation 
of generations. 
" 8. Thus, then, it appears probable that this remarkable phe- 
nomenon may have arisen from the simple circumstance that certain 
animals leave the egg at a very early stage of development : and that 
the external forces acting on the young are different from those which 
affect the mature form. 
" 9. The dimorphism thus produced differs in many important 
respects from the dimorphism of the mature form which we find, for 
instance, in the ants and bees ; and it would therefore be convenient 
to distinguish it by a different name. 
" But there is still another aspect under which, if time had per- 
mitted, the metamorphoses of insects might have been regarded. 
In one or two cases, indeed, I have sketched very briefly and im- 
perfectly the habits and mode of life of particular insects. A whole 
course of lectures might be filled with such life histories. The various 
manners in which different insects provide for the wants of their young 
are most remarkable ; and all the more so because these wants are so 
different from those of the perfect insects themselves. 
" Thus, the butterfly which lives on honey, and did live on leaves, 
lays her eggs on a twig. She seems to feel that honey will not suit 
her young, and that the leaves will wither and fall before another spring 
comes round. 
" The gnat, which lives in the air and feeds on blood, lays her eggs 
on the surface of water ; and the sugar-loving housefly knows that 
very different food is necessary for her young. 
" The nut-weevil chooses the embryo of the nut ; the goat-moth the 
bark of the willow ; the Rhipiphora braves the dangers of the wasp's 
nest ; the OEstrus lays on cattle ; the Ichneumon in caterpillars ; the 
gall-fly in the still almost imperceptible bud ; and some insects even 
in the eggs of others. 
" Generally, the larvae forage for themselves ; but, in some cases, 
the mother supplies her young with food. Thus, the solitary wasp 
builds a cell and fills it with other insects. If, however, she imprisoned 
them while alive, their struggles would infallibly destroy her egg ; 
if she killed them, they would soon decay, and the young larva, when 
