112 A HISTORY OF BIRDS 



died from the number of the glutinous seeds which clogged 

 their feathers. Again, Dr. C. W. Andrews, of the British 

 Museum, has shown that Pisonia seeds are constantly being dis- 

 tributed over Christmas Island (Indian Ocean) by sea-birds 

 whose feathers become so clogged thereby that flight is consider- 

 ably impeded. 



Birds of prey aid in this work of seed dispersal in another 

 way. Vegetivorous fishes, when eaten by piscivorous birds — 

 Hawks, Owls, Kingfishers and so on — ^frequently contain seeds 

 in the stomach. These are either torn from the body of the 

 victim on the margin of the stream, or are ingested by the bird 

 with the prey ; but in either case many germinate, sometimes 

 being deposited far from the place whence they were taken, 

 and from the habits of the bird, the chances that they will be 

 set free in a favourable spot are considerable. Similarly, when 

 granivorous birds are eaten by Hawks and Owls, the seeds 

 contained in the victim's crop are either scattered over the 

 "•round at once, or are sown in the excreta of the slayer. 



Thus, then, it is clear that the influence of birds on the 

 plant world, though not perhaps very great, is by no means a 

 negligible quantity. But some plants, such as the mistletoe and 

 its allies, are dependent for their very existence to the agency 

 of birds, and it seems certain that the brightly coloured 

 fruits of our hedgerows have come into existence through the 

 same cause. That is to say, they have been developed to 

 serve as allurements to bring about seed dispersal and the spread 

 of the species. More correctly, perhaps, the factors which 

 brought about the initial stages in the development of this fruit 

 — whether mutation or the cumulative action of definite varia- 

 tion — secured to these plants material advantages in the struggle 

 for existence ; and hence the increasing perfection in the device, 

 if device it may be called. On the other hand, the birds have 

 also profited, for fruit-eating birds certainly owe their existence 

 to this evolution of fruit-bearing plants. In distributing the 

 seeds of these they are sowing for their future use, as much as 

 when men sow a corn-field. In favour of the mutation theory 

 we may remark that, whatever may have originally served as 

 the inducement to birds to swallow the seeds of these plants, 

 served at the same time, in all probability, to encourage muta- 

 tion, inasmuch as it has been shown that, as a rule, the growth 



