+--------+---------+---------------+---------------------------------+ | | | | Governing Temperatures | | | | +-----------------+---------------+ | | | | NORTHERN LIMIT |SOUTHERN LIMIT | | | | +-----------------+---------------+ | Realms | Regions | Zones | Sum of normal |Normal mean | | of | | | mean daily |temperature of | | Ly- | | | temperatures |the six hottest| | dekker | | | above 6 deg. C. |consecutive | | | | | or 43 deg. F. | weeks | +--------+---------+---------------+--------+--------+-------+-------+ | | | | Deg. C.| Deg. F.|Deg. C.|Deg F. | | | |{Arctic | | | 10[3] | 50[3] | | |{Boreal |{Hudsonian | | | 14[3] |57.2[3]| |Arcto- |{ |{Canadian | | | 18 | 64.4 | | gaeic |{ |{Transition | 5,500 | 10,000 | 22 | 71.6 | | |{Austral |{Upper austral | 6,400 | 11,500 | 26 | 78.8 | | | |{Lower austral | 10,000 | 18,000 | | | |Neogaeic |Tropical |(At present | | | | | | | | unclassified) | 14,500 | 26,000 | | | +--------+---------+---------------+--------+--------+-------+-------+ [3] Estimated from insufficient data. The boundaries of the regions and zones given in the above table are shown on the map facing page 173, but for detailed information concerning the basis of the classification the reader is referred to the monographs by Merriam mentioned at the end of this chapter. In the publications referred to lists are presented of the resident mammals and birds characteristic of each region and of its subdivisions to the north of Mexico. While the boundaries shown on the accompanying map can be recognised in nature by the naturalist and serve a useful purpose, to the unskilled observer each region would appear to blend with its neighbours by intangible gradations. In fact, in this, as in the case of so many other similar instances in nature, there is an absence of definite, or what may be termed hard and fast lines. The significance of the boundaries referred to, to the unskilled observer, is still more obscure by the fact that the migratory birds, and to some extent the mammals, annually pass from one zone to another, and besides, several conspicuous mammals and birds are permanent residents in more than one zone. THE MAMMALS