136 HYPOTHESIS OF THE DEVELOPMENT OP 



kingdom which come broadly under the observation of natu- 

 ralists. In the mollusca there occurs a modifiability of a most 

 remarkable nature. Fresh- water species of these, exposed to 

 brackish water, assume, where able to survive the change, 

 characters in the exterior form of the shell proper to their 

 marine congeners, and involving differences from the original 

 animal much greater than is usually sufficient with naturalists 

 to constitute a distinction of species, if not of tribe or family. 

 Many years ago, Pennant remarked the singular modification 

 of stomach which the common trout appears to have under- 

 gone in the lakes of the county of Galway, in consequence of 

 feeding on shell-fish. The integument has become as thick 

 as the gizzard of a bird, manifestly in consequence of an effort 

 of nature to accommodate herself to the peculiar food of the 

 animal. So also, when a common gull was fed upon corn, 

 the parietes of the stomach were found, on examination after 

 death, to be thickened. ( 67 ) The peculiar forms of the mandibles 

 of birds are grounds of specific distinction ; yet it is now 

 ascertained that these are variable under particular conditions 

 as to food. It has been tried with confined birds ; and even 

 in a wild state there occur individuals strangely modified in 

 this respect, the magpie, woodpecker, and rook, having all 

 been found with the crossed mandibles of the loxia.( 68 ) Look 

 also at the changes from the wild animals to those domesticated 

 ones which are known to be descended from them. " When 

 the eggs of the wild goose," says Professor Low, " are taken, 

 and the young are supplied with food in unlimited quantity, 

 the result is remarkable. The intestines, and with them the 

 abdomen, become so much enlarged, that the animal nearly 

 loses the power of flight, and the powerful muscles which 

 enabled him in a wild state to take such flights, become feeble 

 from disease, and his long wings are rendered unserviceable. 

 The beautiful bird that outstripped the flight of the eagle, is 

 now a captive without a chain." Another change is the 

 transition from grey to white plumage. In the domestication 

 of the pig, the author last quoted admits that there are reduc- 

 tions of the number of teeth, and variations of the number of 

 the dorsal, lumbar, sacral, and caudal vertebrae, producing 



