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NORTH AMERICAN FAUNA. 



speak with certainty, tlie explanation of the fact is probably very sim- 

 ple. Living- throughout the warmer part of the day in cool, dark, and 

 for the most part damp situations, bats, even in widely separated locali- 

 ties, are exposed to comparatively little variation in temperature. 

 Feeding at a distance above the surface of the ground and during the 

 hours between sunset and sunrise, when colors are scarcely distin- 

 guishable, they are practically freed from that necessity for protective 

 coloration which binds the color of most mammals so closely to that of 

 their surroundings. From this reduction in the force of two of the 

 most powerful factors in the production of geographic variation — dif- 

 ferences in temperature and need for protective coloration — the com- 

 parative constancy in the characters of bats naturally results. 



GEOGRAPHIC DISTRIBUTION. 



From the peculiar habits of bats it results that the ranges of these 

 animals are less closely limited by life areas than in the case of most 

 mammals. To be more accurate, the frequent dampness and usual low, 

 even temperature of the retreats occu^jied by bats during the hot 

 part of the day expose the animals to essentially similar conditions 

 wherever they may be, so that a given region of like environment is 

 much more extended geographically for a bat than for most other 

 mammals.^ 



Therefore, although many species seemingly disregard the laws of 

 geographic distribution, their independence is more apparent than real. 



MIGRATION. 



A factor which introduces much uncertainty into the study of the 

 distribution of bats is the little understood migrations which some 

 species are known to make. That many bats migrate is a well-estab- 

 lished fact, but the extent to which migration affects the apparent dis- 

 tribution of species is not known. 



Although there are probably earlier references to the subject, the 

 first mention of bat migration that I have seen is by Dobson, in his 

 Catalogue of the Ohiroptera in the British Museum, published in 1878. 

 In his remarks on the geographic distribution of Pipistrellus ahramus, 

 Dobson says: ''Found during the summer uronths in the Palaearctic 

 region throughout middle Europe; * * * evidently migrates no«'th- 

 ward, * * * as it has never been taken in Europe in winter" (p. 227). 

 In 1888 Dr. 0. Hart Merriam published evidence in the Transactions of 

 the Koyal Society of Canada (V, Section V, p. 85), which showed con- 

 clusively that two American bats, Lasionycteris noctivagans and Lasi- 

 urus vinereus, perform regular periodical migrations. Xo details of the 



'Analogous condiMons are found in sphagnum bogs and heavy, damp woodlands, 

 in which animals of northern affinities, such a& shrews, lemmings, and red-backed 

 mice, extend far south of the normal limit of their kind. 



