376 THE BIOLOGY OF BIRDS 



it may be an increase in constitutional vigour, a lengthening 

 of life, a change in the rhythm of metabolism, a change in 

 the activity of a regulatory gland, an increase or decrease in 

 reproductivity, a change of habit towards activity or sluggish- 

 ness, an experiment in behaviour, the waning or waxing of 

 an instinct. It may also mean the lengthening out of one 

 chapter of the life-history, e.g. brooding, and the telescoping- 

 down or even suppression of another, e.g. nest-making. 



§ 6. Modifications in Birds 



As we have explained, the word " modification " is used 

 in a technical sense by many biologists to denote changes 

 that are wrought on the individual as the direct result of 

 peculiarities in nurture, i.e. in environment and food, in use 

 and disuse. They are dints or imprints which persist after 

 the inducing causes have ceased to operate, but there are 

 very few cases where there is even a hint that they can be 

 entailed on the offspring either as such or in any representa- 

 tive degree. 



John Hunter and others have shown that the walls of the 

 stomach of gulls and some other birds may be experimentally 

 altered by a change of diet. If the food be made harder, the 

 wall of the stomach assumes more of a gizzard character. 

 If the food be made softer, there is a relative increase of the 

 soft part of the stomach. The experiment is made annually 

 in the case of the Herring Gull which has a harder stomach 

 in summer when it is eating a considerable quantity of grain, 

 and a softer stomach in winter when it is subsisting largely 

 on fish. There is unwelcome evidence in the North of 

 Scotland that the Herring Gull is taking more and more to 

 vegetarian diet in the farmer's fields. If we suppose this 

 new habit, fortunately punctuated by the seasons, to become 

 more and more habitual, it is plain that in the course of time, 

 those gulls would succeed best which had greatest plasticity 

 in the temporary adjustment of their stomach-walls to 

 diversified diet. There is no doubt as to the individual 



