422 PHYSIOLOGY. 



the form of the body is somewhat changed; besides, all other Arthrozoa 

 are likewise subjected to this moulting. They differ from the remaining 

 Arthrozoa, namely, from those of the third group, merely by the pre- 

 sence of new organs of motion peculiar to them, and the presence of 

 these organs constitutes really their physiological and philosophical 

 character. But insects with a perfect metamorphosis likewise present 

 this character and a second one in addition, namely, the repetition of 

 all the earlier forms of the Arthrozoa during their period of development. 

 It is a positive fact, confirmed by the history of the development of all, 

 especially of the vertebrata, that the degree of perfection of an organism 

 or organ is the greater the more numerous the grades of development 

 are which it must traverse to attain its full perfection. If we apply 

 this law to insects, it follows incontestably that insects with a perfect 

 metamorphosis must be placed higher in the series of animal bodies than 

 insects with an imperfect metamorphosis. 



We may now ask, why was such a difference of insects from each 

 other necessary ? Why could not all develope themselves, and propa- 

 gate in the same manner ? To this we may reply Nature endeavours 

 to make every possible use of the means which she has conceived 

 allowable for the variation of a determinate type, that is to say, all the 

 forms that are elaborated by the normal progress of development, she 

 absolutely creates and produces as independent creatures. This law, 

 which we find everywhere confirmed, will furnish us with a key to the 

 necessity of a difference among insects with respect to their metamor-* 

 phosis. I refer for this purpose to the four chief classes of the Arthrozoa, 

 each of which is characterised by its place of abode and the possession 

 of peculiarly formed organs of motion, and we already saw above that 

 the presence of wings in any of the Arthrozoa suffices to raise it to the 

 class of insects. But we also perceive that Nature, if she will derive 

 differences merely from the organs of motion, possesses no further 

 means to found new variations, for she has already exhausted the 

 forms of these organs. Whence, then, should she obtain means for the 

 attainment of her object of producing the greatest possible variety, if 

 she did not resort to the last, which is the repetition of the earlier forms 

 in a higher grade of perfection ? She, therefore, avails herself of 

 this, and allows one portion of insects to be distinguished from all 

 the other Arthrozoa merely by the presence of wings, whereas the other 

 portion of already winged insects she raises so above the preceding, that 

 she conducts them, before they arrive at their final stage, through the 



