HABITATIONS OF INSECTS. 451 



by a peculiar fluid, should cause the growth of such .sin- 

 gular protuberances around it, philosophers are as little 

 able to explain, as why the insertion of a particle of va- 

 riolous matter into a child's arm should cover it with pus- 

 tules of small pox. In both cases the effects seem to pro- 

 ceed from some action of the foreign substance upon the 

 secreting vessels of the animal or vegetable : but of the 

 nature of this action we know nothing. Thus much is 

 ascertained by the observations of Reaumur and Malpighi 

 — that the production of the gall, which however large at- 

 tains its full size in a day or two a , is caused by the egg or 

 some accompanying fluid : not by the larva, which does 

 not appear until the gall is fully formed b ; that the galls 

 which spring from leaves almost constantly take their 

 origin from nerves c ; and that the egg, at the same time 

 that it causes the growth of the gall, itself derives nourish- 

 ment from the substance that surrounds it, becoming 

 considerably larger before it is hatched than it was when 

 first deposited d . — When chemically analysed, galls are 

 found to contain only the same principles as the plant from 

 which they spring, but in a more concentrated state. 



No productions of nature seem to have puzzled the an- 

 cient philosophers more than galls. The commentator 

 on Dioscorides, Mathiolus, who agreeably to the doctrine 

 of those days ascribed their origin to spontaneous gene- 

 ration, gravely informs us that weighty prognostications 

 as to the events of the ensuing year may be deduced from 

 ascertaining whether they contain spiders, worms, or flies. 

 Other philosophers, who knew that except by rare acci- 

 dent no other animals are to be found in galls, besides 



a Reaum.iii. 474, b Ibid. 479. c Ibid, 501. d Ibid 479, 

 2g2 



