490 SIR J. LUBBOCK ON THE DEVELOPMENT 
The term, however, has met with much opposition, and is clearly inapplicable to the 
differences exhibited by insects in different periods of their life. Strictly Speaking, the 
phenomena are very frequently not alternate, and, in the opinion of many eminent 
naturalists, they are not cases of generation at all*. 
In order, then, to have some name for these remarkable phenomena, and to distinguish 
them from those cases in which the mature animal or plant is represented by two or 
more different forms, I think it would be convenient to retain for these latter exclu. 
sively the terms dimorphism and polymorphism; and those cases in which animals or 
plants pass through a succession of different forms might be distinguished by the name 
of dieidism or polyeidism. 
The conclusions, then, which I think we may draw from the preceding and other con: 
siderations, are :— | 
lst. That the occurrence of metamorphoses arises from the immaturity of the con- 
tion in which some animals quit the egg. 
2nd. That the form of the larva in insects, whenever it departs from the original 
vermiform type, depends in great measure on the conditions in which it lives. The 
external forces acting upon it are different from those which affect the mature form; and 
thus changes are produced in the young which have reference to its immediate wants, 
rather than to its final form. | 
9rd. That metamorphoses may therefore be divided into two kinds, developmental and 
adaptational. 
4th. The apparent abruptness of the changes which insects undergo arises in great 
measure from the hardness of their skin, which permits no gradual alteration of form, and 
which is itself necessary in order to afford sufficient support to the muscles. i 
5th. The immobility of the pupa or chrysalis depends on the rapidity of the changes 
going on in it. | 
6th. 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. 
7th. When the external organs arrive at this final form before the organs of reproduc 
tion are matured, these changes are known as metamorphoses ; when, on the contrat; 
the organs of reproduction are functionally perfect before the external organs, OF y 
the creature has the power of budding, then the phenomenon is known as alternation 
generations. 
Insects present every gradation, from simple growth to alternation of generations. 
Sth. Thus, then, it appears probable that this remarkable phenomenon may have d 
from the simple circumstance that certain animals leave the egg at a very early stage r 
development, and that the external forces acting on the young are different 10 
those which affect the mature form. | 
9th. The dimorphism thus produced differs in many important respects from the 
ages x = such thing " a true case of 3 alternation of generations” in the animal kingdom ; m: pe^ 
generation with the totally distinct process of gemmation or fission.” — Huxley on Anm 
duality, Ann, & Mag. of Nat. Hist., June 1852. 
