204 GENERATION. 



can only begin at a visible stage of formation, prior to which we are 

 left to conjecture, which could only lead us back to still fewer parts ; 

 but when the first and necessary parts were first formed, as a basis to 

 put the whole succeeding ones into action, so as to increase themselves 

 and form new parts, is not known, nor can it. 



[Magnifying] glasses lead us back far beyond what the naked eye 

 reaches ; but these only show us the order of priority in the formation 

 of parts. However, human wisdom can go no further than into the 

 distinction of parts, with their actions and uses when formed. 



The mode of the gradual increase of the parts of an animal may be 

 considered in three views ; one, where it may be supposed that the 

 basis of eveiy part of an animal is laid at the very beginning, and that 

 its visible perfection is no more than the parts beginning to grow as 

 they are wanted, but that they were there in embryo 1 . Another, 

 where it may be supposed that at first the parts were formed, but were 

 no more in number than just what were wanted for that state of 

 perfection ; and as they came to a degree of perfection, new parts were 

 necessary, and they formed, or formed as they were wanted 2 . And the 

 third is, where the parts were there from the beginning, but that they 

 were altered in form, action, &c. 3 So far as my observations go, I 

 think I can see all the three principles introduced, but probably not in 

 the same animal, nor in the same order of animals. 



According to the first, I can conceive there are, at the very beginning, 

 parts which continue through life, and such is, probably, the Materia 

 Vitce universalis and the Absorbing System, which may indeed, accord- 

 ing to the third principle, be changed. But according to the second, 

 as the embryo is moving towards perfection, new parts are formed ; 

 probably first the brain and heart, with their appendages the nerves 

 and vessels, and so on of all the other parts of the body, which we do 

 not find at first. And we know, according to the third [principle], 

 that many parts are changed in form, adapting their use, arising from 

 that formation, to the addition of parts with the changes in the parts, 

 and this pretty universally. 



Perhaps the flying-insect is the best example of these observations. 

 This insect has three modes of life, and of course three structures of 

 parts. The structure suitable to the first life [ovum] we know little 

 about, but the difference between the second and third we can examine. 

 In the second life [larva] it appears to have no parts but what are of 

 immediate use for the growth of the animal, and some of them very 



1 [The theory of 'Evolution.'] 2 [The theory of ' Epigcnesis.' ] 



3 [The theory of ' Metamorphosis.'] 



