106 



BRITISH BIRDS. 



their nests with a hning when other individuals dispense 

 with it altogether ? The explanation may lie in the 

 undoubted capacity for imitation that birds possess. 

 TPiis would account for the presence of a lining where it 

 was superfluous, or where, as has been shown to be 

 frequently the case, it is too rudimentary to be of the 

 least use. Imitation, even among human beings, is often 

 quite unintelhgent. Or it may be due to a tendency 



Fig. 8. — Common Tern's Nest of Broom at Romney Marsh. 



inherited from some remote ancestor living under different 

 conditions. Before, however, we can balance probabilities, 

 we must decide whether the lined nests are to be regarded 

 as the beginning of an advance, or as a survival, persisting 

 not by virtue of necessity, but simply because it is 

 harmless. The first step towards a solution is to find 

 out whether the mortality among the chicks before or 

 immediately after quitting the egg-shells is due to the 

 absence or presence of a lining. 



The second question may be stated thus : How comes 



