IN THE SHETLANDS II 
the theory I have just laid down, since such generalised 
and legitimate longings are only indirectly related to 
the bird’s special instinct. 
I do not myself see how these curious relations of 
robber and robbed could have arisen, unless there had 
been, from the beginning, a marked difference in the 
relative powers of flight possessed by each. The 
skua, originally, must have caught fish, like the birds 
on whose angling it is now dependent, and only an 
easy mastery over the latter could have induced it to 
abandon the one way of living for the other. This 
superiority was probably first impressed upon the 
weaker species through bodily suffering, but it would 
have been less trouble for the stronger one could it 
have succeeded without coming to extremities, and 
this, and its constantly doing so, might in time have 
made it forget, as it were, the last act of the drama. 
But say that the skua has forgotten this, then it is 
likely that a certain number of the persecuted birds 
have by practice discovered that it has, and so 
emancipated themselves from the tyranny. Whether 
this be the reason or not, I have often noticed the 
persistence with which some terns refuse to yield the 
fish, though the nearness of the skua, and its sweeping 
rushes, seem quite sufficient to induce them to. Those, 
on the other hand, who drop it quickly, often do so 
whilst the enemy is still at a distance, in which case 
the fish falls upon the water before the skua can catch 
it. Upon this, the latter—if not invariably, as the 
fishermen assert, yet certainly in the greater number 
