90 ELEMENTS OF BIOLOGY. 



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 in practice, if it be remembered that they are 

 merely modifications of development, and express differen- 

 ces of a degree and not of kind. An insect, such as a But- 

 terfly, furnishes us with the most striking illustration of what 

 is meant by these terms. All the changes which are undergone 

 by a Butterfly in passing from the fecundated ovum to the 

 condition of an " imago " or perfect insect, constitute its 

 development. The egg laid by a Butterfly undergoes a series 

 of changes which eventuate in its giving birth to a caterpillar 

 or "larva" (fig. 23. a), these preliminary changes constitut- 

 ing its transformation. The caterpillar is totally unlike the 



Fig. 23. Large White Cabbage Butterfly (Pontia brassier}, a Larva or Caterpillar ; 

 b Pupa or Chrysalis ; c Imago or perfect Insect 



adult insect in appearance, and possesses organs which adapt 

 it to a totally different mode of life. It grows rapidly in 



