164 BRITAIN^S BIRDS AND THEIR NESTS. 



The Cormorant, we remember, is a member of the 

 Pehcan Order, so that we naturally compare its fishing 

 and its weapon with those of the Gannet, and, if we 

 may bring in a foreign species, with those of the Pelican 

 itself. The former we have already described, and the 

 latter must be known to everybody. The Pelican's 

 lower mandible is a bag with a stiff but not inflexible 

 rim, and this is used as a sort of surface tow - net, the 

 birds forming in long line, and, gaping wide, sweeping 

 the shallows. Thus we have three allied species fishing 

 in three different ways. Their food is the same, but 

 their methods of obtaining it are fundamentally different. 

 Each method has its appropriate instrument perfectly 

 adapted to its circumstances. The Cormorant pursues, 

 twists, turns, and seizes ; the Gannet soars, plunges, and 

 spears ; the Pelican sweeps and engulfs. 



Conversely, we have unallied birds showing close 

 resemblance in the matter of beaks and similar features, 

 either through coincidence or through adaptation to the 

 same mode of life. Examples abound : Crane and Heron 

 for beaks, with length of neck and legs ; Phalarope and 

 Coot for feet ; Swallow and Swift for beak and wings. 



Thus we realise how much habit has counted for in 

 the evolution of superficial characteristics such as the 

 beak, the feet, and the wings, and we can understand 

 how unreliable such characteristics are as a basis of 

 schemes of classification, and into what errors they led 

 the systematists of a century ago. As a mere system of 

 conventional arrangement and docketing, a classification 

 based on external characteristics might possibly be the 

 most convenient. But this is a secondary and incidental 

 object. The true aim of systematists is to unravel the 

 complicated relationships in the evolution of species, and 

 to build up their genealogical tree. To accomplish this, 



