iv.] METAMORPHOSES OF INSECTS. 79 



The same considerations throw much light on the 

 remarkable circumstance, that in alternation of gene- 

 rations the reproduction is, as a general rule, agamic 

 in one form. This results from the fact that repro- 

 duction by distinct sexes requires the perfection both 

 of the external and internal organs ; and if the phe- 

 nomenon arise, as has just been suggested, from the 

 fact that the internal organs arrive at maturity before 

 the external ones, reproduction will result in those 

 species only which have the power of agamic multi- 

 plication, 



Moreover, it is evident that we have in the animal 

 kingdom two kinds of dimorphism. 



This term has usually been applied to those cases 

 in which animals or plants present themselves at 

 maturity under two forms. Ants and Bees afford us 

 familiar instances among animals ; and among plants 

 the interesting case of the genus Primula has recently 

 been described by Mr. Darwin. Even more recently 

 he has made known to us the still more remark- 

 able phenomenon afforded by the genus Lythrum, 

 in which there are three distinct forms, and which 

 therefore offers an instance of polymorphism. 1 



The other kind of dimorphism or polymorphism 

 differs from the first in being the result of the differ- 

 entiating action of external circumstances, not on the 

 mature, but on the young individual. Such different 

 forms, therefore, stand towards one another in the 

 relation of succession. In the first kind the chain 

 of being divides at the extremity ; in the other it 



1 Of course all animals in which the sexes are distinct are in one 

 sense dimorphic. 



