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 THE VARIABLE DESERT 41

��THE VAEIABLE DESEET

Bt Db. J. ARTHUR HARRIS STATION FOB BXPSBIMBNTAL BYOLUTION, COLO SFBINO HABBOB, N. T.

SINCE 1903, the Desert Botanical Laboratory of the Carnegie Insti- tution of Washington has served as a base for field excursions for many scientific men — ^not botanists merely, but geographers, geologists, physiographers, climatologists, zoologists and anthropologists. These men have come into personal contact with the southwestern deserts and have looked upon inert and living features with eyes trained for the moBt diverse sorts of details. Many of them have set forth in popular as well as in technical terms their impressions of these vast and scien- tifically fascinating regions of our national territory. To add another to the excellent descriptions of the region available from the pens of Coville, McGee, Homaday, Huntington, Lloyd, Spaulding and Mac- Dougal would be quite superfluous were it not for the fact that none of these men have, as it seems to me, emphasized in a few paragraphs the one essential feature of our southwestern deserts, which makes them possibly the best naturally equipped experimental laboratories which have been placed within the reach of American students of living things.

The striking characteristic of this whole region is heterogeneity, variability, contrast — whichever one may wish to call it. This is mani- fest in every fundamental element of the environmental substratum — geographic, physiographic, climatic, edaphic and biologic. Its conse- quences are discernible in every feature of the biological superstruc- ture — ^floristic, morphologic, physiographic and genetic.

To the average reader, the word desert calls up the mental picture of a region of bare rocks and dry wind-swept sand, inhospitable to any but the toughest plant and intolerable to any but the hardiest animal. Such a desert presents to the imagination a landscape ^of the direst monotony — a landscape exactly the opposite of that of our southwestern deserts, which are fiilled with diversity and interest.

Geologically, the deserts of the general region of Tucson consist of a number of rugged mountain groups, varying greatly in age and com- position, with their long detrital slopes and the alluvial valley of the Santa Cruz.

Biologically, the topography of a region is generally a far more im- portant factor than its composition. Within easy reach of the Desert Laboratory are three ranges of mountains with elevations of 9,000 feet or over. Thus within a few miles of the lowest point in the Santa Cruz

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