HABITATIONS OF INSECTS. 451 



of a leaf or twig-, even if accompanied, as some imagine, 

 by a peculiar fluid, should cause the growth of such 

 singular protuberances around it, philosophers are as 

 little able to explain, as why the insertion of a particle 

 of variolous matter into a child's arm should cover it 

 with pustules of small-pox. In both cases the effects 

 seem to proceed from some action of the foreign sub- 

 stance upon the secreting vessels of the animal or ve- 

 getable ; but of the nature of this action we know no- 

 thing. Thus much is ascertained by the observations 

 of Reaumur and Malpighi — that the production of the 

 gall, which however large attains its full size in a day 

 or two% is caused by the egg or some accompanying- 

 fluid ; not by the larva, which does not appear until the 

 gall is fully formed ^ ; that the galls which spring from 

 leaves almost constantly take their origin from nerves "^ ; 

 and that the egg, at the same time that it causes the 

 growth of the gall, itself derives nourishment from the 

 substance that surrounds it, becoming considerably 

 larger before it is hatched than it was when first depo- 

 sited*^. — 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 

 ancient philosophers more than galls. The commen- 

 tator on Dioscorides, Mathiolus, who agreeably to the 

 doctrine of those days ascribed their origin to sponta- 

 neous generation, gravely informs us that weighty 

 prognostications as to the events of the ensuing year 

 may be deduced from ascertaining whether they con- 

 tain spiders, worms, or flies. Other philosophers, who 

 * Reamn. iii. 474. " Ibid. 479. " Ibid. 501. " Ibid. 479. 

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