DEVELOPMENT. 91 



size, but, though it repeatedly changes its skin, it retains its 

 characters for a longer or shorter period. It then ceases to 

 eat, becomes enveloped in a chitinous skin, and loses all its 

 former powers of locomotion. It now constitutes what is 

 known as the ''chrysalis" or "pupa" (fig. 23, b). In this 

 quiescent, motionless, and apparently dead condition it 

 remains for a longer or shorter time, during which develop- 

 mental changes are going on rapidly in its interior. Finally, 

 the chrysalis ruptures, and there escapes from it the perfect 

 winged insect or "imago" (fig. 23, c). To these changes 

 the term metamorphosis is rightly applied. These changes, 

 however, do not differ in kind from the changes undergone 

 by a Mammal ; the difference being that in the case of a 

 Mammal the ovum is retained within the body of the parent, 

 where it undergoes the necessary developmental changes, 

 so that at birth it has little to do but grow, in order to be 

 converted into the adult animal. 



From these considerations we arrive at the generalisation 

 laid down by Quatrefages : " Those creatures whose ova 

 owing to an insufficient supply of nutritious contents, and 

 an incapacity on the part of the mother to provide for their 

 complete development within her own substance are ra- 

 pidly hatched, give birth to imperfect offspring, which, in 

 proceeding to their definitive characters, undergo several 

 alterations in structure and form, known as metamorphoses." 



When the young organism, therefore, is thrown upon the 

 world at a very early period of its development, it generally 

 differs much from the adult in its external characters, and 

 its mode of life is mostly quite different to that of the latter. 

 As a result of this, it commonly happens that the young 

 animal possesses some of the structures of the adult in a 

 very much modified form, whilst it may possess others 

 which are of a merely provisional nature, and are altogether 

 wanting in the fully-grown organism. Thus the caterpillar 

 has to feed upon hard substances, whilst the butterfly lives 

 upon vegetable juices. The caterpillar, therefore, is fur- 



