In Cliloeoi), though the changes are gradual, the difference between 
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 iu 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 larvae of certain Cecidomyias have the 
faculty of producing other larvae, 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 generations ; we see how, 
from a single fact, metamorphoses and alternate generations may have 
originated, and we find reason to suppose that in tlie course of time the 
latter phenomenon may become more frequent than it is at present. 
It is, moreover, evident that there are in the animal and vegetable 
kingdoms two kinds of Dimorphism. The term has generally been 
applied to those cases in which — as in the ant and bee, in animals, and 
the Primulas among plants — the perfect individuals are divided into two 
forms. In fact, the sexes themselves constitute a kind of Dimorphism. 
In these cases the forms are not alternate. When, however, external 
forces act on the young in one manner, and on the mature form in 
another, they tend to produce different forms, which do not complement, 
but succeed, one another. I have elsewhere proposed to distinguish 
this form of dimorphism, under the name of Dieidism or Polyeidism. 
In Polymorphism the chain of being divides at the extremity ; in 
Polyeidism it consists of dissimilar links. 
Finally, the speaker said : — " The principal conclusions which I 
would impress on you this evening are : — 
1. That the presence of metamorphoses in insects depends, in 
great measure, at least, upon the early state in which they quit the egg. 
" 2. That metamorphoses are of two kinds — developmental and 
adaptational. 
" 3. That the apparent abruptness of the changes which they 
undergo arises in great measure from the hardness of their skin, which 
permits no gradual alteration of form, and which is itself rendered 
necessary in order to afford sufficient support to the muscles. 
" 4. That the immobility of the pupa or chrysalis depends on the 
rapidity of the changes going on in it. 
" 5. That although the majority of insects go through three well- 
marked stages after leaving the egg, still a large number arrive at 
maturity through a somewhat indefinite number of slight changes. 
* The instances in which certain insects breed while their wings are but im- 
perfect, 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. 
