DEVELOPMENT. 33 



energies. In no case is this more strikingly exhibited than in 

 the majority of insects, which pass the greater portion of their 

 existence in a sexually immature condition, and die almost 

 immediately after they have become sexually perfect, and 

 have consummated the act whereby the perpetuation of the 

 species is secured. 



ii. DEVELOPMENT, TRANSFORMATION, AND METAMORPHOSIS. 



Development is the general term applied to all those changes 

 which a germ undergoes before it assumes the characters of 

 the perfect individual ; and the chief differences which are ob- 

 served in the process as it occurs in different animals consist 

 simply in the extent to which these changes are external and 

 visible, or are more or less completely concealed from view. 

 For these differences the terms " transformation" and "meta- 

 morphosis " are employed ; but they must be regarded as essen- 

 tially nothing more than variations of development 



Transformation is the term employed by Quatrefages to de- 

 signate " the series of changes which every germ undergoes in 

 reaching the embryonic condition ; those which we observe in 

 every creature still within the egg ; those, finally, which the 

 species born in an imperfectly developed state present in the 

 course of their external life." 



Metamorphosis is defined by the same author as including 

 the alterations which are " undergone after exclusion from the 

 egg, and which alter extensively the general form and mode of 

 life of the individual." 



Though by no means faultless, these terms are sufficiently 

 convenient, if it be remembered that they are merely modifi- 

 cations of development, and express differences of degree and 

 not of kind. An insect, such as a butterfly, is the best illus- 

 tration of what is meant by these terms. All the changes 

 which are undergone by a butterfly in passing from the fe- 

 cundated ovum to the condition of an imago, or perfect insect, 

 constitute its development. The egg which is laid by a butterfly 

 undergoes a series of changes which eventuate in its giving 

 birth to a caterpillar, these preliminary changes constitut- 

 ing its transformation. The caterpillar grows rapidly, and 

 after several changes of skin becomes quiescent, when it is 

 known as a "chrysalis." It remains for a longer or shorter 

 time in this quiescent and apparently dead condition, during 

 which period developmental changes are going on rapidly in 

 its interior. Finally, the chrysalis ruptures, and there escapes 

 from it the perfect winged insect. To these changes the term 



VOL. i. c 



