108 BRITISH BIRDS. 



bird constructing its first nest takes the first " lawful " 

 material that accident places in its way, and so contracts 

 the beginning of a habit that leads it to use normally the 

 same material in the construction of all subsequent nests, 

 even though the search for it demands much more time 

 and labour than taking any other " lawful " material 

 that happens to be near the adopted site. This theory, 

 which would apply equally to cases in which both sexes 

 took part in building, is at first sight plausible enough. 

 It depends, however, upon a question of fact. It should 

 not be difficult either by marking birds or watching their 

 behaviour in captivity to find out Avhether they tend to 

 continue the use of nesting material once adopted. 

 Perhaps someone already has the facts. If so, let him 

 write and deliver. 



Facts it is that are wanted, and as far as nests are 

 concerned, it should not be difficult to collect a large 

 number. Those who are prepared to co-operate in this 

 work will at least have the satisfaction of feeling that 

 their time is being put to good use. The question of 

 variation, specific or individual, structural or functional, 

 occupies to-day a place in the foremost rank of scientific 

 problems, because it takes us to the roots of the evolution 

 theory. It has no mere academic importance. Human 

 progress depends upon human control of natural forces. 

 There can be no control of these forces except by under- 

 standing the laws that govern their operations. And 

 these laws can be reached only through a persevering 

 accumulation of seemingly trivial facts. If there is one 

 thing that Darwin, Wallace, and their successors have 

 made clear, it is the immeasurable importance of the 

 unimportant. They have shown us that from the spectacle 

 of the humble Thrush collecting a beakful of rotten wood 

 for its nest there is but one step to the brink of the un- 

 plumbed depths that hide the answer to the riddle of 

 the universe. 



