100 ESSAY ON CLASSIFICATION. 



became very early the subject of most remarkable obser- 

 vations ; 1 but so little was it then known that all animals 

 undergo great changes, from the first to the last stages of 

 their growth, that metamorphosis was considered a distin- 

 guishing character of Insects. The differences between In- 

 sects in that respect are, however, already found to be so 

 great, that a distinction has been introduced between those 

 which undergo a complete metamorphosis, that is to say, 

 which appear in three different successive forms, as larvae, 

 pupae, and perfect insects, and those with an incomplete 

 metamorphosis, or whose larvae differ little from the perfect 

 insect. Yet the range of these changes is so limited in 

 some insects, that it is not only not greater, but is even 

 much smaller than in many representatives of other 

 classes. We may, therefore, Avell apply the term meta- 

 morphosis to designate all the changes which animals 

 undergo, in direct and immediate succession, 2 during their 

 growth, whether these changes are great or small, pro- 

 vided they are correctly determined for each type. 



The study of Embryology, at first limited to the inves- 

 tigation of the changes which the chick undergoes in 

 the egg, has gradually extended to every type of the 

 animal kingdom ; and, so diligent and thorough has been 

 the study, that the first author who ventured upon an 

 extensive illustration of the whole field, C. E. von Baer, 

 has already presented the subject in such a clear manner, 

 and drawn general conclusions so accurate and so com- 



1 SWAMMERDAM (J.), Biblia Natu- 2 I say purposely, "in direct and 



rae, sive Historia Insectorum, etc. ; immediate succession," as the pheno- 



Lugduni-Batavorum, 1737-38, 3 vols. mena of alternate generation are not 



fol., fig. REAUMUR (R. ANT. DE), included in metamorphosis. They con- 



Memoires pour servir a 1'Histoire des sist chiefly in the production of new 



Insectes; Paris, 1734-42, 6 vols. 4to., germs, which have their own rueta- 



fig. ROESEL VON ROSENHOF (A. J.), morphosis; while metamorphosis pro- 



Insectenbelustigungen ; Niirnberg, per relates only to the successive 



1746-01, 4 vols. 4to. ; fig. changes of one and the same germ. 



