THE SWALLOWS AGAIN. 49 



tropical insects find their food in the large bells or deep 

 tubes of the brilliant equatorial flowers. So the swifts 

 would naturally take to flitting about in the neighbour- 

 hood of these blossoms and poising themselves on their 

 powerful wings just in front of their corollas. Those of 

 them which took permanently to such a mode of life 

 would soon adapt their external structure to the new 

 conditions with which they had grown familiar. Tro- 

 pical swifts with the longest bills and the most extensile 

 tongues would have an advantage over others, because 

 they would best be able to probe the long tubes of the 

 flowers and extract the insects from them, inside the 

 nectary itself. In this way the bill and tongue have 

 gradually grown so long in their descendants, the hum- 

 ming-birds, that all outer resemblance to the parental 

 swallow form has been w^holly lost ; and the family was, 

 accordingly, classed till quite recently with the extern- 

 ally similar, but genealogically quite distinct, group of 

 sun-birds. 



In most other respects, however, the humming-birds 

 continue to resemble the ancestral swifts. The shape of 

 the wing and its proportion to the body is exactly the 

 same ; but, above all, the numerous minute anatomical 

 points of similarity settle the question at once for modern 

 biology. Even before evolutionism gave the new key 

 which solves so many of these difficult problems, it was 

 noticed that the humming-birds were very like the 

 swallows in many anatomical particulars, though very 

 unlike them in plumage and in the shape of the bill. 

 Dr. Jerdon, who has spent his life in studying the birds 

 of India, hesitated about ranking the sun-birds by their 

 side because of this structural community between 



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