Some Salient Points in the Science of the Earth

by Sir John William Dawson

[197] Wollaston.

Now this is precisely what seems to have occurred in the Pleistocene period. The Arctic land remained in great mass, detaching into the sea annual crops of icebergs and fields of coast ice, which have strewed all the northern hemisphere with boulders: the temperate regions were submerged, except a few insular spots. These are the very conditions required for a low mean temperature, both in the sea and on the land, and these geographical conditions correspond precisely with the facts as indicated by the fossil animals and plants of the period. We must bear in mind, however, that under certain contingencies the high mountain summits might have been clad in snow and ice, like Greenland, and the Alpine plants might have been able to live only on their margins.

Further, it would be easy to show that the Alpine plants of Mount Washington would thrive under such conditions as those supposed, at the sea level; a low and equable temperature, with a moist atmosphere, being that which they most desire, and their greatest enemy being the dry parching heat of the plains of the temperate regions. Those of them, such as Potentilla tridentata and Alsine Grœnlandica, which occur in low ground within the limits of the United States, are found under shaded woods, in damp ravines, or on the moist sea-coast; and as we follow the coasts northward, we find these plants, on these and on neighbouring islands, in lower latitudes than those in which they occur inland. This is well seen in Northern New Brunswick and in the south shore of the St. Lawrence, where several northern species occur in shady and moist localities. I have, for example, collected Cornus Suecica and the Alpine birch in such places. When the summer mists roll around the summit of Mount Washington, it is in every respect the precise counterpart of an islet anywhere on the coast of America, from Cape Breton to the Arctic seas, and when winter wraps everything in a mantle of snow, all these lands are in like manner under the same conditions. So, in the Pleistocene period, though the islets of the White Mountains may have experienced a less degree of winter cold, they must have had very nearly the same summer temperature as now; and as this is the season of growth for our Alpine and Arctic plants, it is its character that determines the suitableness of the locality to them.

Those stupendous vicissitudes of land and water which have changed the aspect of continents, and swept into destruction races of gigantic quadrupeds, have dealt gently with these Alpine plants, which long ages ago looked out upon a waste of ice-laden waters that had engulfed the Pliocene land with all its inhabitants, as securely as they now look down upon the pleasant valleys of New England. It is curious, too, that the humbler tenants of the sea have shared a similar exemption. In the clay banks of the Saco, on the shores of Lake Champlain, and mixed with the remains of these very plants in the valley of the Ottawa, are shells that now live in the Gulf of St. Lawrence and on the coast of Maine, intermixed with other species that are now found only in a few bays of the Arctic seas. Just as in the Post-pliocene clays of the Ottawa, the remains of northern plants are found in the same nodule with those of Leda glacialis, so now similar associations maybe taking place on the coasts at the mouth of the Great Fish River. Truly, in nature as in grace, God hath chosen the weak things of the world to confound those that are mighty, and has left in the earth's geological history, monuments of His respect and regard for the humblest of His works.

It is interesting to notice here that Greenland, at the present time, presents conditions as to vegetation which may, in some respects, correspond to those of the White Mountains in Pleistocene times. Its flora, though altogether Arctic, contains 386 species, none of which are peculiar to it, but many of them range quite round the Polar circle. Of those that are not so generally distributed, some, more especially on the west coast, are common to Greenland and Arctic America. Others, and a larger number, more especially on the east coast, are common to Greenland, Iceland and Norway, between which and Greenland there may have been a closer land connection than now, in Pliocene and Post-glacial times.

We look in vain among the Alpine plants, so long isolated in these mountains, for any evidence of decided change in specific characters. The Alpine plants, for ages separated from their Arctic brethren, are true to their kinds, and show little tendency to vary, and none to adapt themselves to new forms in the sunny plains below. This is especially noteworthy on Mount Washington and the neighbouring peaks, because the soil of these is the same with that of the valleys. Several of the plants peculiar to these hills, as the black crowberry (Empetrum nigrum), for instance, even when other conditions are favourable, shun rich calcareous soils, and affect those of granitic origin. In many cases the difference in soil is a sufficient reason for the non-occurrence of such plants, except on certain hills. At Murray Bay, and on the shores of Lake Superior, the plant above named occurs only on the Laurentian gneiss. In Nova Scotia, its relative, Corema Conradi, is confined to the granite barrens of the south coast. Many such plants skirt the whole Laurentian range from Labrador to Lake Superior, but refuse to extend themselves over the calcareous plains of Canada. But in the White Hills the soil of the river alluvium is the same micaceous sand that fills the crevices of the rocks in the mountains, and hence there is no obstruction, in so far as soil is concerned, to the diffusion of plants upward and downward in the hills. In like manner there is every possible condition as to moisture and dryness, sunshine and shade, in both localities. These circumstances are of all others the most favourable to such variation as these plants are capable of undergoing. The case is the same with that which Hugh Miller so strongly puts in relation to the species of algæ that occur at different distances below high water mark on the coast of Scotland, each species there attaining a certain limit, and then, instead of changing to suit the new conditions, giving place to another. So it is on Mount Washington; and this, whether we regard the lowland plants that climb to a certain height, and there stop, the plants that are common to the base and summit, or the plants that are confined to the latter.

I have already referred to the evident struggle of the spruces and firs, and the plants associated with them, to ascend the mountain, and the same remark applies to all the plants that one after another cease to appear at various heights from the lower valleys. One by one they become stunted and depauperated, and then cease, without any semblance of an attempt to vary into new and hardier forms. And this must have been proceeding, be it observed, from all those thousands of years that have elapsed since the elevation of the mountains out of the glacial seas. It is to be observed, also, that the new plants that occur in ascending, often belong to different genera and families from those left behind, not to closely allied species; and in the few cases in which this last kind of change occurs, there is no graduation into intermediate forms. For instance, Solidago thyrsoidea and S. virga-aurea[198] occur around the base of the mountain, and for some distance up its sides. At the height of four to five thousand feet the latter only remains, and this in a dwarfish condition. This corresponds to its distribution elsewhere, for, according to Richardson, it occurs in lat. 55° to 65° in Arctic America, and according to Hooker, it is found in the Rocky Mountains, while it also occurs in the hills of Scotland, and very abundantly in some parts of Norway. In the White Mountains S. thrysoidea prevails toward the base, S. virga-aurea toward the summit; and at the top of Tuckerman's ravine I found the former of these golden rods in blossom, within a few hundred feet of the latter, each preserving its distinctive peculiarities. Much has lately been said of the appearance of specific diversity that results from the breaking up of the continuity of the geographical areas of plants by geological changes; but here we probably have the converse of this. The mountain species is no doubt a part of the older Arctic flora, the other perhaps belong to a more modern flora, and they have met on the sides of the White Hills.

[198] Macoun thinks that most of the specimens referred to this species belong to the allied form, S. Mulllinallata, Ast, which is very extensively distributed on the mountains of British America and in the Arctic regions.

Some hardy species climb from the plains to heights of 5,000 feet or more, with scarcely even the usual change of being depauperated, and then suddenly disappear. This is very noteworthy in the case of two woodland plants, the dwarf cornel or pigeon-berry (Cornus Canadensis), and the twin-flower (Linnæa borealis). The former of these is a plant most widely distributed over northern America, and probably belongs to that newer flora which overspread the continent after its re-elevation. In August this plant in the woods around the base of Mount Washington is loaded with its red berries. At an elevation of four to five thousand feet it may be found in bloom; above this a few plants appear, destitute of flowers, dwarfish in aspect, and nipped by cold, and then the species disappears. No doubt the birds that feed on its little drupes have carried it up the mountain, and have sown it a little farther up than the limit of its probable reproductiveness. The beautiful little Linnæa is a still more widely distributed plant; for it occurs on the hills of northern Europe, and is found across the whole breadth of the American continent from Nova Scotia to the Columbia River. It is almost beyond question a member of the old Arctic flora which colonized the islands of the Pleistocene sea, and 'has descended from them on all sides as the land became elevated. This plant also climbs Mount Washington to a height of 5,000 feet, and presents precisely the same characters on the top as at the bottom, only losing a little in the length of its stem. Specimens bearing blossoms, and quite in the same stage of growth, may be collected at the same time on the highest shoulders of Mount Washington, and on the flats at Gorham. The Linnæa in this is true to its designation. For, as if it belonged to it to support the reputation of the great systematist after whom it is named, it preserves its specific characters with scarcely a tittle of change throughout all its great range. One cannot see this hardy little survivor of the Glacial period, so unchanging yet so gentle, so modest yet so adventurous, so wide in its migrations yet so choice in the selection of the mossy nooks which it adorns with its pendant bells, and renders fragrant with its delicious perfume, without praying that we might, in these days of petty distinctions and narrow views, be favoured with more such minds as that of the great Swede, to combine the little details of the knowledge of natural history into grand views of the unity of nature.