176 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



of this organ still await investigation, but sufficient is 

 now known as to its importance to lead us to hope for 

 much instructive information as to the action of its 

 secretion in different animals. 



Although it is absolutely certain that structural 

 characters are profoundly modified by changes in 

 function, or, in the words of John Hunter, " I dare 

 say the different manner of living gives rise to the 

 different formation of the viscera," it is, on the other 

 hand, a fact beyond contradiction that in two purely 

 herbivorous animals, such as, for example, the horse 

 and the cow, or piscivorous forms as the seal and the 

 porpoise, we find anatomical structures which are 

 strikingly different ; to understand this we must again 

 invoke the principle which, as we have already said, 

 stands equal in value to that of the power of varying 

 with varying circumstances ; certain modifications of 

 structure are impossible to certain animals, on account 

 of the influence of heredity ; in other words, descent 

 as much as environment has to be taken into account 

 in the study of the morphological characters of the 

 parts of any organism. 



The first definite evidence as to the influence of 

 food on the structural characters of the digestive canal 

 was given by John Hunter, when he fed a sea-gull for 

 a year on barley, and found that the muscular tissue 

 of the gizzard became enormously developed. 



It is stated that "this experiment is annually 

 repeated by Nature; that the herring-gull, Larus 

 tridactylus, of the Shetland Islands, twice every year 

 changes the structure of its stomach according to its 

 food, which consists during the summer of grain and 

 in winter of fish." Somewhat similar observations 

 have been made on the raven and the owl, and the 

 converse experiment, or that of converting the stomach 

 of the grain-eating pigeon to the carnivorous type, has 

 been effected by Holmgren. 



