*

Now it is a remarkable fact that the white sands, thirty or forty miles off to the northeast, exhibit an almost identical flora. The student hastens across the intervening desert to meet that shining wall, expecting to find all things new; but, behold, the white sands are sands first of all rather than anything else. Whatever their chemistry, and they have their peculiar problem for the chemist, only a vegetation that can endure a moving, shifting terrene can flourish here. The white sands form, accordingly, part of the yucca desert. Their relation to vegetation is almost purely physical, but they exhibit some peculiarities. They are gypsum, as everybody knows, but while they move as other sands, they must be compared with wet sands; the vast drifts, thirty to fifty feet in height, are moist often to within a few inches of the surface, and are so compactly driven that one may walk upon the solid surface with comparative ease. A white wall like to the appearance of marble is moving slowly eastward, whelming all vegetation as it goes, some of which, able to grow through the encroaching mass, persists, so that all the plants now appearing on the surface, so far as examined, are anchored by lengthened stems or roots to the underlying older soil. The same yucca that appears at Escondida here emerges sometimes by green tips from a snow-white drift twenty feet in height, or anon, seems to crown triumphantly some lower mound. The mesquite holds on, in some places a desperate fight, and certain species of Rhus -R. aromatica and R. trilobata, perhapsmaintain a perilous existence out over the

*The following analysis of this material has been kindly furnished me by Dr. L. W. Andrews of the Mallinckrodt Chemical Works, St. Louis: Calcium sulphate, CaSO,... Water, H.O...

Calcium carbonate, CaCO1.

77.64 per cent. 20.55 0.95

whole region, sometimes even on the summits of the highest knolls. These sumacs are the characteristic species of the white sands.

But let us turn north. A journey of fifteen or twenty miles brings us to the black wall of the lava flow. This is a fearful region. The Mexicans call it mal pai, 'bad country'; giant floods whose waves are stone, fields and fissures, caverns, holes, pits and wells, alternating with tilted slopes, knife-edge culms and ridges, make a topography weird, impassable, fascinating because so unapproachable. Yet the mel pais is covered with vegetation. Of course, the vegetation changes, but by no means as one might easily suppose. Here is no new species, no variety of a species when the desert is studied as a whole. The change is correspondent to a change in level. The lava beds are high, and they are crowned with the flora of their own altitude. We shall meet it on the foothills of all the mountains we presently ascend. Here is no alteration of soil, for the only soil is that deposited by the wind, the lava itself being perfectly intractable. Here are the familiar mountain cedar, Juniperus occidentalis; cholla, sometimes twelve or fifteen feet high, where, springing in some ragged well-hole, it seems to peer out above the sooty walls that hem it in; here is the mountain barberry. Even the nut pine, Pinus edulis, has mistaken these pitchy steeps for the clayey flanks of its usual mountain fastness, and now and then rivals the cedar in its hold upon the jagged upturned edges of these flinty sheets. Even the lava beds have not apparently affected the general character of the desert flora.

At the south end of these black fields, however, emerge great springs. Here all the plain is saturated with salt and alkali, and here is a peculiar flora conditioned by this fact. The waters emerge almost from the edge of the lava sheets, and tufts of

Suada and Allenrolfia are set close against the lava wall. This is ideal; this we should expect and here it is.

The sands and the lava lie in the middle of our desert. If we take these as a starting-point and move toward the summit of the mountains, the successive belts of vegetation gradually shape themselves so that we learn presently to identify them by their color. A plain below the general level is gray, grass-covered, with here and there a bunch of Ephedra or nopal, no yuccas, no Atriplex, no other forms of cactus. As the terrene rises to the silt plain, thickets of cholla alternate with mesquite and the crucifixion thorn; not that other species do not occur, but these are dominant, give to the belt its character and color. A little further mountainward and we reach the Covillea tridentata, ever in bloom, which lies as a girdle of green and gold around the whole base of the mountain range, visible for miles and marking for us the limits of the talus with an exactness that is remarkable. Beyond the Covillea belt come the cacti as the terrene becomes more rocky; Mamillaria, with its species numerous and varied, the unique but widely distributed ocatillo, the prickly pear, often in giant form—all these cover the rocky slopes that lead up to the steeper walls of paleozoic rocks. Sometimes, where a shelf occurs, and the bare limestone forms a flat, mesa-like field, the yuccas come back, but not the Escondida form, with Agave parryi, and abundant ocatillo, while in the rocky defile below, locked amid gigantic boulders, now on their tardy journey to the talus plain, the creamy flowers and fruit of Dasylirion lift their glorious spikes, the envy and vexation of the photographer.

The strata of the lower carboniferous limestones now confront us; crystalline, encrinitic and exceedingly hard, rising often hundreds of feet sheer up and down. But these dry walls likewise have their

flora. Mamillaria micromeris matches with its hoary spheres the weathered stone or lights it up betimes with scarlet bloom, and Notholana innata fills with somber tufts every shattered crevice.

But the upper members of the carboniferous are much softer and, amenable to erosion, present a gentler, flowing topography. These slopes are everywhere clothed with oak, not trees indeed; far from it; low dense shrubs, the so-called shin-oak, Quercus gambellii and Quercus gunnisoni. These two species form pale green belts around the mountains, and are recognized easily, distinguishable for miles. These species indeed form a sort of phytographic border land; all below is desert; all above is forest; for above stands, or lately stood, one of the fairest bits of woodland in the United States, and that means in the world. But this forest is again in large measure conformable to geologic structure, its distribution determined by the history of what lies beneath.

As we ascend the mountain, passing all the carboniferous limestones, sands, chalkbeds and shales, we presently encounter the 'red beds' already mentioned, the most remarkable geological horizon in the country, familiar to every student of our central mountains, noted even by the ordinary tourist, the same wherever found-in Utah, Colorado, the Black Hills of South Dakota, and here again in these far-off mountains of the Mexican border, the same vast gypsum-burdened deposits of clay and shale. and sand. The red beds yield easily to erosion. The washings from their wasted flanks have tinged the desert far below, and reddened the walls of every rocky cañon on the way. Sloping terraces and flat-topped hills afford a soil rocky but not infertile, supporting once more its own peculiar vegetation. Here are still the shinoaks, it is true, but all overshadowed by other nobler trees; here is Berberis trifolio

lata, the Texan barberry; here is Pinus edulis, Engelmann's nut pine, and most characteristic and perfect of all, here stands Juniperus pachyphlæum, the mountain juniper, great forests of it, ancient trees betimes, all comparatively low, but with giant trunks six or eight feet in diameter; these time-defying cedars are the trees of the red beds. With the junipers, especially as we pass their upper limits and come out upon the calcareous cretaceous swells and plains, occurs another oak or two. The soils are now remarkably rich in lime. The waters that fall on the higher mountain levels escape above the red-bed shales, but so impregnated with lime that they actually form a new stony deposit often for a distance of many rods about the point of exit. On these calcareous soils stands now the forest, along the very summit of the mountain, nine thousand feet above sea. level, a magnificent forest of spruce and pine and fir: Pseudotsuga douglasii, the Douglas spruce, five or six feet in thickness; Abies concolor; Pinus ponderosa in beautiful perfection of its immortal youth; Pinus flexilis at its very best; a typical Oregon forest six or eight miles wide and some twenty long, crowning the summit of this isolated mountain peak in the midst of the deserts of southern New Mexico, for, as everybody knows, these are in general species of the forest of the far Pacific coast. As one stands now at last thus at the very summit of his problem, and from some promontory rock of vantage looks out upon the vast plain thus mountain-girt, the indescribable beauty of the scene must first impress him. Far to the west lie the San Andreas, the Organ and the Oscuro ranges, a long low wall, gray and solid, its serrate summits indentured in the azure sky; below, the plain, brilliantly lighted, soft and brown and lucid, save as the mal pais stretches its blackness as a bar sinister across the northern end, while away to the

south the gypsum desert seems a cloud of snow beneath our feet, more brilliant than that evanescent whiteness that floats in the deep blue far above-the one the strange counterpart of the other; all is so silent, so changeless and so fair!

But just now we heed not the beauty of the landscape; other thoughts come crowding upon the observer, all equally insistent and impressive. Evidence of enormous physical change thrusts itself upon our astonished attention; not the sunken desert itself alone, that great block already described, but the denuded and sundered mountain walls, the great cañons that stretch back for miles, cut down through even the solid limestones at the mountain base-a process vast and old. Once the cretaceous sea rolled here, and when it retreated here were beds of limestones hundreds of feet thick. Where are they now? Only here and there a remnant on the mountain summit; the desert is covered with their débris almost to distant sea.

No less is one impressed by the slowness of all this topographic change. There is evidence of violence, suddenness, nowhere, save in the mal pais, which is local, recent, and does not affect the general problem. The moving currents of the air, the soft ministrations of the summer shower, the melting winter snows, have carved these mountains, are sculpturing them to-day. Those columnar whirlwinds that even now like dancing dervishes chase each other across the plain, are shaping anew the desert; that thin cloud that hangs yonder like a banner from the mountain top is a rainstorm, changing even now the general altitude of the range.

But once again; as we look out thus from the summit of our problem we are impressed with still another fact more farreaching, more splendid still. The whole living covering of the world, the vegetative garment of the desert and the mountain,

conforms exactly to the surface, to soil and level, no doubt with an exactness that we have only begun to guess or understand. There is a mathematical line that limits the distribution of every plant, but the area forever shifts and varies. The topography varies, except the mal pais, by changes so slight, so delicate, as to be imperceptible to eyes unskilled, and with the topography varies its covering of life. Let us say first that these topographic changes will change the limits of distribution. Once the sands cover the silt plains, and the grasses will vanish while yucca and artemisia succeed. Widen the talus and covillea will stretch farther its golden scepter. But the problem runs far deeper than this. As the face of the world undergoes these delicate, subtle changes, the plant responds in something far more than shifting distribution. A plant, as every student of botany well knows, is the most plastic sort of an organism in the world, responding in every sort of way to its environment. We who study the microscopic structure of the humblest plants understand the limitless possibilities here. When we reflect that the suppression of a single cell at the critical moment may change the direction of the axis or alter the contour of a leaf, it is hard to set too high an estimate upon the possible response made by a simple plant to environmental variations, however delicate. We who study the physiology of the plant, peer into its changing cells and strive in imagination to reproduce the marvelously intricate reactions, physical, chemical, that forever shift and play within those narrow limitswe need not be told that every vegetable cell has in it opportunities a thousandfold to match and meet all the subtle changes suggested by the slow-creeping but implacable forces that work out the physiognomy of this time-worn earth. A little more calcium here, a little more phosphorus there,

sulphates, nitrates and the rest, and the thing is done. Nay, when we even think of the form in which all energy comes from yon distant sun, and the delicate machinery on which it plays, we need seek no further occasion for the intervention of every sort of outer cosmic force. Not a tree on all the Iowa prairies but shows in its every lineament, in its very expression, a response to the Iowa environment; and so, we may be sure, every desert plant records in its present form and stature all the affirmations, all the responses it has made in all the centuries to the bidding, the silent bidding, the most gentle coaxing, of the world external. For, note you, the call for change at any given instant has not been great; the slow upheaval of these mountains, their peaceful, gentle removal by the winds and rain; that is all; but that has changed and is changing the living world. Where the terrestrial call is rude or sudden, response there is none. The lava beds show no single characteristic species. Their flora is simply that of their own rocky level. Nor could here any sudden initiative on the part of the plant avail. The adaptation is absolute now, and to vary save as the environment varies would simply invite disaster. As well the tadpole suddenly assume lungs or the lizard put on feathers.

Nor is this all-our desert as it lies shining here before us is but a fraction of that wider, vaster desert that covers all the south and west. Across the Organ and San Andreas yonder is another desert exactly comparable to that we study; all Arizona, southern California, Sonora, Chihuahua, much the same; here and there a mountain summit tufted with forest, western in type, high slopes thinly clad with stunted juniper, benches of covillea, wide low plains covered with mesquite, with yucca and cactus and all the less noble plants that stand between; and our prob

lem widens, becomes vast as the continent, and any answer that we make must be farreaching as the flora of a world.

Our desert lies shining here before us; but not one of these plants except the cactus is in broader sense unique; each has its kin rising in happier fields to fairer fortune. The yuccas are lilies, but lilies bloom in Bermuda and in Teneriffe, and in every most fertile garden of the world. The mesquite is a Prosopis, but the Prosopis genus shows many a handsome forest tree, and even the mesquite in the Arizona valleys, where conditions are less hard, rises a forest with trees fifty feet in height. cactus, as I read it, with undifferentiated floral leaves and abundant sporophylls, is an ancient adaptation to an ancient desert, possibly pre-cretaceous, and takes possession of the world just so fast as the world becomes desert; unstable in cultivation, not because new, but because reversionary.

The

I do not mean to say necessarily that the Alamogordo desert flora has had its origin where it stands, although such a contingency is not impossible of thought. Had this been the only desert on the continent its flora is as might have been expected. But there are a hundred similar intra-montane regions whose geologic history is the same. These have in similar fashion originally shaped a flora each for itself. No doubt once similar conditions are set up in regions at first unlike, an exchange of species may take place. American cacti are at home in the deserts of Europe and the Russian thistle flourishes on Dakota plains.

The desert lies shining here before us, changing forever, but all its changes are of imperceptible delicacy and slowness. Its methods would seem not different from those by which nature has from the first essayed the education of the vegetable world. Between salt water and fresh all conditions offer by infinitesimal shadings where the rivers meet the sea, thus green

plants first emerged from ocean; all conditions from shore-line low-water mark to dry land, thus the plants at length sat on the shore, wet only by tides or by the gentle rain; all conditions of level by which the plants occupy the kingdom of the upper air: all conditions of spore-union by which they meet at length the problem of aerial fertilization: so that while sports there may be among plants outside the pale of cultivation, nevertheless, they must always be within limits set as result of more gentle changes effected by the slow, and for the most part exquisitely delicate, transformations which make up the history of the planet. Given a desert flora, a cactus flora, for instance, and there may be endless species-making, by sport, if you will, or otherwise, but in every case a cactus; but the cactus itself is the child of continental movements which brought about some oldtime, perhaps cretaceous desert.

Our desert lies shining before us; it is old and silent would you know its secret, read the rocky records that lie behind, around, beneath, and be assured that once the story of yesterday were understood, the facts of to-day would ask no wider explanation. The physical forces of this world still drive the loom that weaves the web of life. Before the loom the unseen weaver sits, guiding her web that passes to an endless roll, changing withal the width, the pattern, as conditions rise. Changes her arabesque, it is for cause, changes it not, it is alike for cause; and if at intervals, as we watch, anon new figures rise, may it not be but the return of some earlier triumphant cycle that here begins anew, evident enough in cause and feature were once that giant scroll unrolled, or were her watchers more patient, more enduring. Alas! in presence of this mighty loom what fleeting, evanescent interpreters are we!

THOMAS H. MACBRIDE,