II VISCERAL VARIATION 105 



stomach of this animal, which, being a flesh-eater, does 

 not require to have the hard and horny coating of the 

 gizzard of the pigeon or fowl. This experiment is 

 repeated each year in nature, and without man's 

 operation, by another gull (Larus argentatus}, of the 

 Shetland Islands, which, according to Dr. Edmonstone, 

 changes the structure of its stomach twice every 

 year, according to its food, which consists of grain 

 during part of the year, and of fish during the other 

 months. So the stomach may vary considerably in 

 its use and functions, and Holmgren's experiments 

 show that the gizzard of a grain-eater, such as the 

 pigeon, may be converted into a carnivorous stomach, 

 such as that of one of the birds of prey. 



I have already said that there is great variability in 

 the muscular system. Some anatomists have made 

 a special study of this variability : Wenzel Griiber 

 in Germany, Testut in France, Cunningham of 

 Dublin, and many others. Not only are there 

 variations in the mode of attachment and course of 

 every muscle of the human body which has been 

 more especially studied in this connection but super- 

 numerary muscles are often found which are all exactly 

 similar to muscles which normally exist in lower 

 animals, but do not as a rule exist in man. Testut 

 has dwelt upon this fact, which is of great significance 

 in the evolution theory, and a very large number of 



