Coloration of the Mouths and Eggs of Birds. 269 



This was found to be as true in relation to wild birds as 

 to captives. Was it also true of the birds themselves, 

 regarded as prey ? I experimented fairly exhaustively with 

 more than a hundred species of birds on a cat, a lemur, and 

 (less fully) an owl and a butcher-bird. In view of the 

 relative size of the prey, I did not expect to find the " grading" 

 at all fine ; yet it was. Using meat-scraps from the different 

 species, I found, as in the other case, every gradation from 

 Z, only eaten when the animal was exceedingly hungry, 

 right up through all the levels of growing repletion to A, 

 accepted at all times up to repletion. Substituting the whole 

 bird for the scrap of its meat, the same thing would occur. 

 If the animal had refused the meat-scrap it would refuse 

 the whole bird too. If it were easily hungry enough for the 

 scrap, it would commonly tackle the bird itself, and might, 

 appetite growing with eating, go on to make a full meal off it ; 

 yet, if it had been offered the same bird when only slightly 

 fuller, it might have refused it absolutely. It was evidently 

 a matter of relative digestibility and varying digestive power, 

 a flow of the digestive secretions being stimulated when the 

 stomach was empty by objects that were untempting, or 

 even, as experiment showed, definitely inhibitive on a some- 

 what fuller stomach. 



Obviously, if the above be the general rule (and I have 

 so far found no exceptions to it), there can be relatively few 

 species of animals that will not sometimes require to be distin- 

 guished by an enemy not hungry enough for themselves, from 

 species (including, often, it may be, their own parent form) 

 that he is hungry enough for. This suggests the contributory 

 explanation for distinctiveness and diversity that I have 

 referred to above. The necessity for differentiation from 

 a pleasanter parent form will have been not the least 

 important consideration, for unless correlated with some 

 new distinctive character, a variation in the direction of 

 increased unpleasantness will hardly have been selected. 

 The cumulative action of this need for differentiation, where 

 oft-repeated in the history of a species, might even be 

 invoked to assist in the explanation of certain cases of 



