380 Royal Institution :— 
erations ; we see, from a single fact, how metamorphoses and _ alter- 
nate generations may have originated, and we find reason to suppose 
that in the 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 ren- 
dered 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. 
“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 
metamorphoses ; 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 alternation of generations. 
“« Insects present every gradation, from simple growth to alter- 
nation 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. 
