SECT. XXXIX. 4. 7. GENERATION. 



the kangaroo, from the fize of a fheep to that of a rat. Many 

 fifh feemed to partake of the (hark ; fome with a fkait's head 

 and fhoulders, and the hind part of a (hark ; others with a 

 fhark's head and the body of a mullet j and fome with a fhark's 

 head and the flat body of a (ting-ray. Many birds partake of 

 the parrot ; fome have the head, neck, and bill of a parrot, with 

 long (Iraight feet and legs ; others with legs and feet of a par- 

 rot, with head and neck of a fea-gull. Voyage to South Wales 

 by Captain John Hunter, p. 68. 



7. All animals therefore, I contend, have a fimilar caufe of 

 their organization, originating from a (ingle living filament, en- 

 dued indeed with different kinds of irritabilities and fenfibilities, 

 or of animal appetencies ; which exift in every gland, and in 

 every moving organ of the body, and are as efTcntial to living 

 organization as chemical affinities are to certain combinations of 

 inanimate matter. 



If I might be indulged to make a fimile in a philofophical 

 work, I (hould fay, that the animal appetencies, are not only per- 

 haps lefs numerous originally than the chemical affinities j but 

 that like chefe latter, they change with every new combination ; 

 thus vital air and azote, when combined, produce nitrous acid ; 

 which now acquires the property of diflblving filver ; fo with 

 every new additional part to the embryon, as of the throat or 

 lungs, I fuppofe a new animal appetency to be produced. 



In this early formation of the embryon from the irritabilities, 

 fenfibilities, and afibciabilities, and confequent appetencies, the 

 faculty of volition can fcarcely be fuppofed to have had its birth. 

 For about what can the fetus deliberate when it has no choice of 

 objects ? But in the more advanced ftate of the fetus, it evident- 

 ly pofleflfes volition ; as it frequently changes its attitude, though 

 it feems to deep the greateft part of its time ; and afterwards 

 the power of volition contributes to change or alter many parts 

 of the body during its growth to manhood, by our early modes 

 of exertion in the various departments of life. All thefe facul- 

 ties then conftitute the vis fabricatrix, and the vis confervatrix, 

 as well as the vis medicatrix of nature, fo much fpoken of, but 

 fo little underftood by philofophers. 



8. When we revolve in our minds, firft, the great changes, 

 which we fee naturally produced in animals after their nativity, 

 as in the production of the butterfly with painted wings from, 

 the crawling caterpillar ; or of the refpiring frog from the fub- 

 natant tadpole ; from the feminine boy to the bearded man, and 

 from the infant girl to the laclefcent woman ; both which 

 changes may be prevented by certain mutilations of the glands 

 neceffary to reproduction. 



VOL. I. C c c Secondly, 



