PRINCIPLES OP ANIMAL DEVELOPMENT. 205 



different in form from what they are afterwards, while others remain 

 the same : so that in the insect we have, in the second life, parts that 

 were probably of nse in the first ; we have, at least in the second, 

 parts that are of use in the third, therefore do not change, such as the 

 brain, nerves, and circulation l ; but in the third life [pupa], we have 

 new parts entirely, and old ones changed. The new parts formed are, 

 the parts of generation 2 , legs, wings, &c. «fec; parts changed are, the 

 whole of the digestive powers, in some degree the organs of respira- 

 tion, and probably the organs of sensation 3 . Thus in the progress of 

 growth, in the more perfect animals, we have new parts arising, changes 

 taking place in those already formed, and old parts lost. 



It may be observed, that the more perfect the order of animals is, it 

 comes to a larger size at the first-life than those of an inferior order : 

 thus, a new-born quadruped is nearer to the size of the parents than a 

 bird just hatched, and a bird nearer than a fish, &c. However, there 

 are varieties in this respect in the quadruped, for some have several at 

 a time, which renders them smaller. 



From this account we should suppose that a quadruped would be the 

 first for investigation ; but as Nature gives to every order of animals a 

 mode of reproduction peculiar to itself, we are led to examine this 

 process in those where its operations are most easily and certainly come 

 at. This must certainly be the case with some of the oviparous, 

 although not with all, and according to the above position the Bird 

 must be the best, and still more so in those that have fewest young in 

 number and largest in themselves. 



Without this aid our knowledge of this subject would have been very 

 imperfect, and it would almost appear that this mode of propagation 

 was intended for investigation. 



In the investigation of this subject they have commonly had recourse 

 to the common fowl, as being the most familiar ; but I found the first 

 appearances so obscure, from want of size in the object, that I had 

 recourse to the progress of the chick in the egg of the goose. I 

 attempted the swan, but it was impossible to procure such numbers as 

 to give me all the necessary varieties. I endeavoured to procure 

 ostrich's eggs, by having them sent to me in spirits ; but as the getting 

 such was only a matter of chance, and only one or two in thirty years ! 

 nothing could be made out from them. For this purpose, then, I kept 



1 [These parts can only be said not to be essentially changed, but they are remark- 

 ably modified in form.] • 



2 [Hunt. Preps. Nos. 3025-3028.] 



3 [See Heroldt, Die Entwickelungs-geschichte der Sehmetterlinge, anatomisch und 

 physiologisch bearbeitet, 1815.] 



