SCHMIDT: ANIMAL GEOGRAPHY 783 



part on considerations that are clearly ecological. In the development of this 

 aspect of biogeography on scientific principles, the botanists took an early lead, 

 and the whole field, for botany, was summarized by A. F. W. Schimper in his 

 important book Pflanzengeographie auf physiologischer Grundlnge in 1898 (Eng. 

 ed., 1903; 3d German ed. 1935). Thus ecological plant geography was summarized 

 in an authoritative way more than a quarter-century before the appearance of 

 the work by Richard Hesse, Tiei-geograpMe auf oekologischer Grundlage, which 

 appeared in 1924. This has now been translated by myself, and quite completely 

 revised by W. C. Alice and myself, for its successive American editions, 1937 

 and 1951, and it is still by no means as comprehensive a treatment of the ecological 

 aspects of zoogeography as is Schimper 's work for phytogeography. 



If we search for the roots of ecological animal geography in North America, 

 it is at once evident that they lie in the development of a systematic zoology 

 focused on the existence of subspecies. As long as species were being described 

 from isolated specimens, there might be very little knowledge of their actual geo- 

 graphic ranges; but at the next level of analysis, the very idea of partitioning 

 the species into subspecies required a definition of the ranges of both; and from 

 such knowledge the step was easy to further ecological analysis of the meaning 

 of the geographic ranges, and of analysis of the factors limiting such range. A 

 school of description of subspecies grew up in North America hand in hand with 

 the ambitious project of C. Hart Merriam for a biological survey of the continent. 

 This in turn was directed into a broadly zoogeographie aspect by Merriams' 

 development of the life zone theory (1890, 1898), in which the correspondence 

 of altitude zones of mountains with the transcontinental climatic zones was 

 pointed out, with an elaborate explanation of their temperature limitation. 

 Merriam 's theory does not bear critical examination, though it was maintained 

 for more than thirty years, and the work of the great United States Biological 

 Survey was set in the Life Zone framework. Fortunately, the question of the 

 existence of life zones is quite independent of the problems of their explana- 

 tion. Merriam thought that northward distribution is limited by the sum of 

 the positive temperatures (defined as degrees above an assumed physiological 

 zero of 6°C) during the entire season of growth and reproduction, whereas 

 southward limits were set by the mean temperature of a brief period during 

 the hottest part of the year. That this was an extreme oversimplification is now 

 evident; it is necessary to consider maximum and minimum temperatures; average 

 temperature of the coldest part of the year; length and temperature of the frost- 

 less season; amount of rainfall; degrees of atmospheric humidity and wind move- 

 ment; day length microclimates; the considerable variety of edaphic factors; 

 topographic barriers; and especially the complex of influences introduced by the 

 sum of the favorable and unfavorable biotic factors. A thoroughgoing critique 

 of the theory has been supplied by the botanists Livingston and Shreve (1921) 

 and the zoologists Dice (1923), Kendeigh (1932), Shelf ord (1932), Dauben- 

 mire (1938), and Pitelka (1941), to which list many more names might be added. 

 The idea of temperature summation, originated as an ecological technique by 

 Reaumur in 1735, is by no means to be discarded as useless — it remains as a 

 measure of available heat supply in the growing season, and as one among the 

 many factors that an ecological biogeographer must examine. 



Even the summed temperatures finally listed by Merriam have been found to 



