4©2 GENERATION. Sect. XXXIX. 5. 1. 



objects and their interftices. The eggs of hedgebirds are green- 

 ifh with dark fpots *, thofe of crows and magpies, which are feen 

 from beneath through wicker nefts, are white with dark fpots ; 

 and thofe of larks and partridges are ruflet or brown, like their 

 nefts or fituations. 



A thing Hill more afbonifhing is, that many animals in coun- 

 tries covered with fnow become white in winter, and are faid to 

 change their colour again in the warmer months, as bears, hares, 

 and partridges. Our domefticated animals lofe their natural 

 colours, and break into great variety, as horfes, dogs, pigeons. 

 The final caufe of thefe colours is eafily underftood, as they 

 ferve fome purpofes of the animal, but the efficient caufe would 

 feem almoft beyond conjecture. 



Firit, the choroid coat of the eye, on which the femitranfpa- 

 rent retina is expanded, is of different colour in differnt ani- 

 mals ; in thofe which feed on grafs it is green ; from hence 

 there would appear fome connexion between the colour of the 

 choroid coat and of that conftantly painted on the retina by the 

 green grafs. Now, when the ground becomes covered with 

 fnow, it would feem, that that action of the retina, which is 

 called whitenefs, being conftantly excited in the eye, may be* 

 gradually imitated by the extremities of the nerves of touch, or 

 rete mucofum of the fkin. And if it be fuppofed, that the ac- 

 tion of the retina in producing the perception of any colour 

 confilts in fo difpofmg its own fibres or furface, as to reflect 

 thofe coloured rays only, and tranfmit the others like foap-bub- 

 bles ; then that part of the retina, which gives us the perception 

 of fnow, mull at that time be white j and that which gives us 

 the perception of grafs, muft be green. 



Then if by the laws of imitation, as explained in Section XII. 

 33. and XXXIX. 6. the extremities of the nerves of touch in 

 the rete mucofum be induced into iimilar action, the fkin or 

 feathers, or hair, may in like manner fo difpofe their extreme 

 fibres, as to reflect white ; for it is evident, that all thefe parts 

 were originally obedient to irritative motions during their 

 growth, and probably continue to be fo *, that thofe irritative 

 motions are not liable in a healthy itate to be fucceeded by fen- 

 iaticn ; which however is no uncommon thing in their difeafed 

 (late, or in their infant ftate, as in plica polonica, and in very 

 young pen-feathers, which are (till full of blood. 



It was (hewn in Section XV. on the Production of Ideas, that 

 the moving organ of fenfein fome eircumllance* rcfembled the 

 object which produced that motion. Hence it may be conceiv- 

 ed, that the rete mucofum, which is the cxtrcmitv of the nerves 

 I touch, may by imitating the motions of the retina become 



coloured. 



