366 MENTAL EVOLUTION IN ANIMALS. 



with cakes of a brittle mucilage which, like that of the 

 esculent swallow, swells and softens in water ; in flame it 

 crackles, swells, does not readily burn, and emits a strong 

 animal odour : it differs only in being yellowish-brown, in not 

 having so many large air-bubbles, in being more plainly 

 laminated, and in having even a striated appearance, caused 

 by innumerable elliptical excessively minute points, winch I 

 believe to be drawn-out minute air-bubbles. 



Most authors believe that the nest of the esculent swallow 

 is formed of either a Fucus or of the roe of a fish ; others, I 

 believe, have suspected that it is formed of a secretion from 

 the salivary glands of the bird. The latter view I cannot 

 doubt, from the preceding observations, is the correct one. 

 The inland habits of the Swifts and the manner in which the 

 substance behaves in flame almost disposes of the supposition 

 of Fucus. Nor can I believe, after having examined the 

 dried roe of fishes, that we should find no trace of cellular 

 matter in the nests, had they been thus formed. How could 

 our Swifts, the habits of which are so well known, obtain roe 

 without being detected ? Mr. Macgillivray has shown that 

 the salivary crypts of the Swifts are largely developed, and 

 he believes that the substance with which the materials of its 

 nest are fitted together, is secreted by their glands. I cannot 

 doubt that this is the origin of the similar and more copious 

 substance in the nest of the North American Swift, and in 

 those of the Collocalia csculenta. We can thus understand 

 its vesicular and laminated structure, and the curious reti- 

 culated structure of the Philippian Island species. The only 

 change required in the instinct of these several birds is that 

 less and less foreign materials should be used. Hence I con- 

 clude that the Chinese make soup of dried saliva.* 



In looking for a perfect series in the less common forms 

 of birds' nests, we should never forget that all existing birds 

 must be almost infinitely few compared with those which 

 have existed since footprints were impressed on the beach of 

 the New Red Sandstone formation of North America. 



If it be admitted that the nest of each bird, wherever 

 placed and however constructed, be good for that species 



* [It is almost needless to observe that we must remember the date at 

 winch this was written; but it may be remarked that as early as 1817 it was 

 pointed out by Home {Phil. Tram., p. 3:$2) that (he proventrieulus of Collo- 

 c ilia is a peculiar glandular structure probably suited to secrete the substance 

 of which the nest consists. — Or. J. 11. j 



