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. 
