80 ON THE ORIGIN AND [CHAP. 



is composed of dissimilar links. Many instances of 

 this second form of dimorphism have been described 

 under the name of alternation of generations. 



The term, however, has met with much opposition, 

 and is clearly inapplicable to the differences exhibited 

 by insects in various periods of their life. Strictly 

 speaking, the phenomena are frequently not alternate, 

 and in the opinion of some eminent naturalists they 

 are not, strictly speaking, cases of generation at all. 1 



In order, then, to have some name for these remark- 

 able phenomena, and to distinguish them from those 

 cases in which the mature animal or plant is repre- 

 sented by two or more different forms, I think it would 

 be convenient to retain exclusively for these latter 

 the terms dimorphism and polymorphism ; and those 

 cases in which animals or plants pass through a suc- 

 cession 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 considerations, are : 



1. That the occurrence of metamorphoses arises 

 from the immaturity of the condition in which some 

 animals quit the egg. 



2. That the form of the insect larva depends in 

 great measure on tl)e 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, having refer- 



1 "There is no such thing as a true case of 'alternation of genera- 

 tions in the animal kingdom;' there is only an alternation of true 

 generation with the totally distinct process of gemmation or fission." 

 -HuxLEY on Animal Individuality, Ann. and Mag. of Nat. Hist. 

 June 1852, 



