372 MENTAL EVOLUTION IN ANIMALS. 



sides are thicker and very different materials are used. In such cases, as was 

 formerly remarked with respect to corporeal structures, a great and apparently 

 abrupt change might be effected in the instinct of a bird by one form alone 

 of the nest being retained. 



In some cases, when the same species ranges into a different climate, the 

 nest differs ; the Artamus sordidus in Tasmania builds a larger, more com- 

 pact, and neater nest, than in Australia (Gould's "Birds of Australia"). 

 The Sterna minuta, according to Audubon (''Anns, of Nat. Hist.," vol. ii, 

 1839, p. 462), in the southern and middle U. States merely scoops a slight 

 hollow in the sand ; " but on the coast of Labrador it makes a very snug nest, 

 formed of dry moss, well matted together and nearly as large as that of the 

 Turdus migratorius. ,> Those individuals of Icterus Baltimore (Peabody in 

 " Boston Journ. of Nat. Hist.," vol. iii, p. 97) " which build in the south 

 make their nests of light moss, which allows the air to pass through, and 

 complete it without lining ; while in the cool climate of New England they 

 make their nests of soft substances closely woven with a warm lining." 



Habitations of Mammals. — On this head I shall make but 

 few remarks, having said so much on the nests of Birds. 

 The buildings erected by the Beaver have long been cele- 

 brated ; but we see one step by which its wonderful instincts 

 might have been perfected, in the simpler house of an allied 

 animal, the Musk Eat {Fiber zibethieus) which, however, 

 Hearne* says is something like that of the Beaver. The 

 solitary Beavers of Europe do not practise, or have lost the 

 greater part of their constructive instincts. Certain species 

 of Eats now uniformly inhabit the roofs of houses,f but other 

 species keep to hollow trees — a change analogous to that in 

 swallows. Dr. Andrew Smith informs me that in the unin- 

 habited parts of S. Africa the hysenas do not live in burrows, 

 whilst in the inhabited and disturbed parts they do.:j; Several 

 mammals and birds usually inhabit burrows made by other 

 species, but when such do not exist, they excavate their own 

 habitations. § 



In the genus Osraia, one of the Bee family, the several 

 species not only offer the most remarkable differences, as 

 described by Mr. F. Smith|| in their instincts ; but the indi- 

 viduals of the same species vary to an unusual degree in this 

 respect ; thus illustrating the rule, which certainly seems to 



* JFearne's Travels, p. 380. Hearne has given the best description (pp. 

 227-230) ever published of the habits of the Beaver. 



+ Rev. L. Jenjns in Linn. Trans., vol. xvi, p. 106. 



X A case sometimes quoted of Hares having made burrows in an exposed 

 situation (Anns, of Nat. Hist., vol. v, p. 302), seems to me to require verifica' 

 lion : were not the old rabbit-burrows used ? 



§ Zoology of the Voyage of the Beagle, "Mammalia," p. 90. 



|| Catalogue of British Hymenoplera, 1855, p. 158. 



