266 BOMBAY DUCKS 
in form, yet both are equally successful in obtaining 
food, and both secure it in the same manner. More- 
over, the young male has a tail four inches in length, 
but, later on, he grows one sixteen inches long, yet he 
continues to obtain food in the same manner. Thus 
a difference of twelve inches in the length of his tail 
does not appreciably affect his ability to find food. 
Even if we could demonstrate that each species takes 
the shape best suited to its mode of life, if we could 
prove, for example, that the Nilgiri blue flycatcher 
would be greatly handicapped in the search for food 
were his shape that of the grey-headed flycatcher, this 
would not be sufficient. If natural selection alone is 
responsible for the shape of an organism, we must 
prove that every step in the transition from the common 
ancestral form to that of the present species was a 
distinct gain to the species. This point is often lost sight 
of by those who invoke the aid of natural selection to 
explain every zoological difficulty. It seems to me that 
the great diversity in shape exhibited by birds having 
similar habits merely shows that there are several 
equally good methods of accomplishing an object. 
If Nature desires to call into existence a number of 
flycatching birds, she is not obliged to cast all in exactly 
the same mould; she is able to create many different 
forms of organism, all well adapted to the work before 
them. The general shape is of course determined by 
natural selection, especially in the case of highly 
specialized birds, such as woodpeckers, kingfishers, and 
swifts. But, even in such cases, considerable diversity 
of form is permitted. The less specialized the habits 
