THE SHAPES OF BIRDS 267 



of a bird are, the greater is the latitude as regards 

 shape allowed to it. 



The shape of organisms is due to the action of a 

 large number of forces, of most of which we are totally 

 ignorant. Natural selection does not interfere unless 

 the variation in shape tends to benefit or injuriously 

 affect the possessor. In the former case, the beneficial 

 shape tends to be perpetuated and to cause the species 

 to spread at the expense of other less-favoured ones. 

 In the latter case the injurious variation leads to the 

 extermination of the creatures in which it appears. 



Natural selection, like the stone walls of a labyrinth 

 of lanes, marks certain limits within which variations as 

 regards shape may persist. So long as the variations 

 are such as do not affect the mobility of a species, its 

 ability to obtain food, or its relationship to its environ- 

 ment, natural selection does not in any way interfere. 



The causes which have produced this diversity of 

 shape among allied species and genera have yet to be 

 discovered. We are not at present in a position to say 

 why some birds are large and others small, why some 

 are slim and others stout, why some have pointed 

 wings and others round ones, why some have broad 

 heads and others narrow ones. It is useless to pretend 

 that natural selection explains all these phenomena. It 

 is better to be honest and frankly admit our ignorance. 



