864 



SCIENCE 



[N. S. Vol. XXXI. No. 805 



is of smaller size, and still smaller is it when 

 present on the hillsides. The distribution, as 

 indicated by its size, is evidently indicatory of 

 the different amounts of available soil mois- 

 ture. The reviewer has noted that large mes- 

 quite occurs on hillsides in Zaeatecas, where 

 there are hidden springs, as indicated by an 

 actual outflow some distance away. 



The mesquite in respect to water-supply is 

 a physiological type to which belong, e. g., 

 Koeherlinia spinosa, Holacanthus sp. The 

 water relations of these plants have given rise 

 to a saying in Mexico: Donde hay junco, hay 

 aqua, " where the junco occurs, there also is 

 water," upon which faith many a dry well has 

 been dug. This a propos of the occurrence of 

 mesquite in the flood plain. 



Of the more distinctly desert associations is 

 Spalding's creosote-bush (Larrea) association. 

 This is almost coincident with the mesa-like 

 slopes of low gradient so characteristic of 

 desert regions. Untoward physical conditions 

 are here — a soil with little capacity for water 

 retention, and underlaid by an impervious 

 hardpan of caliche. To the most rigorous of 

 these conditions the creosote-bush is the last 

 to succumb, and is often the only plant with a 

 perennial foothold. 



The peculiarities of local distribution con- 

 tingent upon the aspect of slopes, especially 

 the steeper ones, have been extremely well 

 studied by Professor Spalding, and the maps, 

 made in detail and accuracy hitherto un- 

 equaled, by Mr. J. C. Blumer, to record ob- 

 servations, rather than merely to illustrate 

 the principles involved, are in themselves a 

 noteworthy contribution. Five species have 

 been thus studied in detail. Of these, the 

 most compelling example, by virtue of its size 

 and appearance, is the sahuaro, Cereus (now 

 Carnegiea) giganteus. This principally af- 

 fects the southern aspects of the hills, the 

 " optimum physical habitat " for this plant. 

 The author has endeavored in this, as in the 

 other cases treated, to refer this peculiar dis- 

 tribution to an efficient cause or set of causes. 

 The search for these has led Professor Spal- 

 ding to very important conclusions. Thus, the 

 choice of habitat is, in many cases, condi- 



tioned by " difference in habit, and power 

 of accommodation," leading to a fixation in 

 particular situations. On the other hand, 

 some plants are distinguished by a wide ca- 

 pacity for adjustment, and hence the restric- 

 tions upon choice of habitat are less strait 

 and insistent. Here is pointed out that physi- 

 ological adjustment may be of far more im- 

 portance than structural " adaptation," but it 

 appears — and this is of major importance — 

 that in both cases " inherited peculiarities de- 

 termine the limits of choice." Apparently the 

 evidence does not indicate a progressive 

 (racial) change in adaptation, but that a 

 chance pre-fitness determines the possibilities 

 of getting along. 



Of chief importance appears to be the 

 " range of temperature, though other factors, 

 in certain cases at least, are involved." A 

 constructive criticism at this point may be 

 made that temperatures may be of this degree 

 of importance in only a secondary way, but 

 this also in certain cases. The view seems 

 justified that the differences of insolation, and 

 so of the temperatures, on slopes of opposite 

 aspect, is effective in selection as between 

 plants, which, during germination, quickly 

 attain a sufficient (and again inherited) de- 

 gree of structural or physiological resistance 

 and those which are slow in this regard. The 

 conclusions before us strongly indicate the 

 great importance of the study of seedling de- 

 velopment, and it may be believed that much 

 light will thus be thrown on many still obscure 

 questions of distribution. 



Nevertheless, Professor Spalding makes a 

 strong case for the direct effect of tempera- 

 ture, as e. g., in the case of the sahuaro, whose 

 limits of distribution appear to be set by 

 temperature limits. It would be of the great- 

 est interest and profit to compare, for this 

 plant, its temperature environment, e. g., in 

 the Sta. Catalina Mountains and those of 

 its present, generally northern, geographical 

 range. 



The so well-known individual isolation of 

 desert plants has given force to the idea very 

 generally accepted, that their interrelations 

 are of minor importance. Pause is given to 



