DIFFERENTIAL FORMS. 107 



localities frequented, and the position of the 

 flowers which are most attractive. Many species, 

 like our swallow, are fitted either for darting 

 through the air at a great altitude or close 

 along the flowery plain. But the wings of 

 the YERYAIN HUMMING-BIRD, which flits like a 

 bee from flower to flower, over the pasture lands, 

 and along banks and hedgerows, would not fit it 

 for dashing round the rocks and pinnacles of the 

 Cordilleras, or for passing,like the RUBY-THRO AT, 

 from Northern Canada to the Intertropics, and 

 back again, on a long migratory expedition. 



Those who have gained a familiar acquaintance 

 with a certain number of the species of the Tro- 

 chilidfje, can at once distinguish between them 

 while on the wing ; as readily, in fact, as we can 

 between our different species of Hirundo. To a 

 practised ear, the very murmur of the wings is 

 sufficient. 



With respect to gorgets, crests, and orna- 

 mental plumes, there is no limit to variety ; here 

 nature appears to indulge in most fantastic 

 luxuriance. We can scarcely suppose that these 

 ornaments are not destined to answer some im- 

 portant purpose, but we are ignorant as to what 

 that purpose is. 



As much, as in ornaments, does variety prevail 

 in the form and length of the beak ; but here we 

 can trace, to some extent at least, a definite 

 reason ; peculiar kinds of flowers require a differ- 

 ent kind of probe ; and hence, as this probe is the 



