38 SEA-BIRDS 



type; and this linear distribution makes it easy to apply Huxley's 

 concept that the characteristics of animals tend to grade from one 

 part of their range to another in an orderly way. Some of these grada- 

 tions had been recognised long before Huxley thought of the word 

 "cline" because they are adaptations to the environment. For instance 

 Bergmann's Rule states that from the warmer parts of an animal's 

 distribution-area to the colder parts there tends to be an increase in 

 its size. Thus the puffins, black guillemots and eider-ducks of the 

 Arctic are considerably bigger than those of Britain. The main adaptive 

 reason for this is that larger animals have less surface in proportion 

 to their weight, and consequently heat is not lost from them (if warm- 

 blooded) so rapidly as it is from small animals. Another rule, Allen's 

 Rule, states that warm-blooded animals of cold climates tend to have 

 their heat-radiating surfaces decreased by a reduction in size of their 

 extremities and limbs such as ears, tails, necks, legs and noses. There 

 is also a general tendency (Gloger's Rule) for animals to become 

 darker as humidity increases. 



If we examine those sea-birds which are widely distributed, we 

 find clines in various characteristics, notably in size, i.e. total size, 

 and also size of limbs and extremities, beak-length, wing-length, 

 etc., and in colour. There are also clines in shape; for instance the 

 fulmars of the north-east Atlantic have very thick bills, those of 

 Baffin Island rather more slender bills, those of the North Pacific 

 more slender bills still, and those of the Antarctic very slender bills 

 indeed. No sea-bird is arranged quite evenly in its geographical 

 distribution. Just as the distribution in space is never even, so are the 

 gradations in character never even. From one part of the geographical 

 distribution of a species to the other, change often occurs more as 

 a series of steps rather than as continuous ramp. 



Most working ornithologists today will agree that there are more 

 subspecific names about than a true understanding of bird evolution 

 requires. It is the species which has reality and significance. In this 

 book we have tried to be sparing in the use of subspecies, and have 

 rejected some that appear in many current text-books. Nevertheless, 

 a study of the geographical races of the species of the North Atlantic 

 sea-birds will lead us to examine here some of the more fascinating 

 examples of geographic differentiation. The classic example among 

 the sea birds is the chain of the Larus argentatus and fuscus group, the 

 herring-gulls and lesser blackbacks, which may include some birds 

 which are regarded as separate species, e.g. the California gull L. 



