DISTRIBUTION OF ANIMALS 87 



Were they not so familiar to us, we should consider it to be 

 impossible that warm-blooded, active creatures, with a bony 

 skeleton, could have their fore-limbs (or arms) so modified 

 as to be used exclusively for flight, and yet, with no organ 

 of prehension but the mouth prolonged into a beak, some- 

 times aided by a foot, be completely adapted to obtain 

 every kind of vegetable or animal food, to protect themselves 

 from enemies, and to construct the most perfect abodes for 

 their helpless young to be found among the higher animals. 



Some zoologists consider that in the power of flight 

 birds are surpassed by insects, but I cannot think this to be 

 the case. If we take into consideration the weight they 

 have to carry, the height they often attain above the earth, 

 their perfect command over the direction and speed of their 

 motion, and the exquisite and highly complex organ by 

 which flight is effected, birds must take the higher place. 

 The insect's flight is simpler and more automatic ; that of 

 the bird more elaborate in every part, more completely 

 under the control of the creature's will. It is also, I believe, 

 more varied in exact adaptation to the mode of life of each 

 of the species. 



As regards their variety of structure, the numbers of the 

 species, and their mode of distribution over the earth's 

 surface as compared with the other forms of life already 

 considered, a few examples will be sufficient to prove their 

 general correspondence with other animals. It must be 

 remembered, however, that in birds the numbers inhabiting 

 the several countries are less precise and less comparable 

 than in any other group. This is due to several causes. 

 In all extra-tropical lands a large proportion of the species 

 are migratory, and the facts observed are very similar over 

 the whole of the north temperate zone. Some go to more 

 northern lands in summer to breed, returning south in 

 autumn ; others leave us in autumn to winter in the south, 

 returning to us in the spring ; others, again, are birds of 

 passage only, staying with us a few days or weeks on their 

 way north or south. All these are considered to be truly 

 natives, in our case to be " British birds." But others only 

 visit us occasionally, some at very long intervals, while others, 

 again, are mere " stragglers," who have lost their way or been 



