210 Notices and Descriptions of various new [No. 159. 



weight of these nests are produced annually ? To myself, who, long ago, 

 following the accounts of the edible nests being constructed by a true 

 Hirundo, found this a stumbling block to one of the distinctions 

 which I drew between the Swallows and the Swifts, I confess it yielded 

 some gratification to find my suspicions in this matter completely con- 

 firmed ; for the nest of Cypselus apus of Europe is essentially similar 

 to that of Collocalia esculenta, containing a large quantity of glutinous 

 matter, which there can be no doubt is secreted by the very large sali- 

 vary glands of the bird* ; whereas in Hirundo urbica, the nests of which 

 species might be thought to present a marked analogy, the fabric is con- 

 structed of mud, or, as Vieillot remarks, worm-casts are selected for the 

 purpose, and the birds may be commonly seen on the ground collect- 

 ing material of the kind, many of them often resorting to the same wet 

 place, — the Swifts, on the contrary, never descending to the ground 

 at all. The two groups of Swallows and Swifts present a very 

 remarkable instance of what is termed analogy, or mere external 

 and superficial resemblance, as opposed to affinity, or intrinsic phy- 

 siological proximity. Though externally resembling in their adaptive 

 characters, as a Cetal may be said to present a superficial resemblance 

 to a fish, sufficient indeed to have occasioned the group to be still popu- 

 larly classed with fishes, the difference between the Swifts and Swallows 

 is analogous in kind, but inferior in degree, to that which necessitates 

 the Whales and Porpoises to be removed altogether from among fishes : 

 and the same intrinsical similarity in the essential structure, which com- 

 pels us to arrange the Cetals in the class of mammalia, equally approxi- 

 mates the Swifts to the Trochilidce (or American Humming birds), 

 while the Swallow conformation is modelled on the ordinary passerine 

 type, from which it deviates only in external modifications, having re- 

 ference to mode of life. In the Swift, as in the Humming bird, the 

 entire structure, alike as regards the rudimental anatomy and the ex- 

 ternal characters, concurs to produce the maximum of volar power ; 

 whereas n the Swallows there is no such general concurrence; but the 

 potency of flight seems entirely due to the development of the wings 

 and tail, the sternal apparatus in no respect differing from that 



* Vide Mag. Nat. Hist. 1834, p. 463 et seq. The nests there described passed into my 

 possession, which enables me to state that the glutinous matter was in greater quan- 

 tity than would appear from the account given by Mr. Salmon. The fact is, it con- 

 stitutes the basis of a Swift's nest, by which is made to adhere the various light sub- 

 stances gathered in the air by these birds, when such are blown about on a windy day. 



