xii INTRODUCTION. 



needed light on the significance of the preen-gland. As 

 Mr Pycraft points out in his delightful and valuable 

 History of Birds (1910), many difficulties confront the 

 view that birds use the secretion of the preen-gland as 

 a sort of lubricant feather-oil. Many birds that have a 

 well-developed oil-gland ' could not possibly take up and 

 spread so much as one drop of the precious fluid."" The 

 Scissor-bill {Rhyncops) with its paper-knife-like jaws may 

 serve as an example. On the other hand, many birds 

 that have no oil-gland — e.g. many Pigeons and Parrots — 

 keep their feathers in first-rate order. These are two 

 out of the many difficulties that confront the orthodox 

 view, and we need not mention more, for our point 

 here is not to argue about this particular problem. We 

 merely take this as an instance of the many gaps in 

 our knowledge, and of a kind of gap which it should 

 not be very difficult to fill. 



Another instance may be found in connection with 

 the sense of smell in birds. In most cases it seems to 

 be weakly developed ; in some cases, such as the South 

 American Vultures, it seems to be quite absent ; only in 

 a few cases, such as the Blackbird and the Owl, has it been 

 experimentally proved. Yet it is well known that many 

 birds seem to be extremely sensitive to the odour of the 

 human hand, forsaking their nest if the eggs have been 

 handled. It ought to be possible to solve a simple 

 problem of this sort by devising a few experiments and 

 carrying them out with patience. 



External AcxivrriES. 



Another line of inquiry, more accessible perhaps to 

 the field-naturalist, opens up when we pass from the 

 internal economy of the body to the external activities 



