76 SYSTEMATIC DISTRIBUTION. 



All the variations exhibited in the colouring of the 

 different species, or in the different individuals of the 

 same species, are produced by the permanent or tem- 

 porary interruption of the action of these glands. 

 But for a more detailed account of these phenomena, 

 I must refer the reader to my papers on the subject 

 in the Philosophical Transactions for 1833. 



