Page:Popular Science Monthly Volume 30.djvu/90

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78
THE POPULAR SCIENCE MONTHLY.

America. The lowest series of Tertiary rocks in Europe contains remains of three species, and the upper series of no less than forty-three species. The hickory seems to have appeared in Europe in the middle Tertiary, and in the upper beds is represented by twenty-one species. In America the walnut is well represented in the lowest Tertiary, and increases in numbers toward the top; while the hickory is represented by several species through the whole series. When the living species of these two genera are considered, a widely different state of affairs appears. The forty-three species of walnuts dwindle to one; the hickories are entirely absent, though perhaps represented by three somewhat different genera now growing in Japan and parts of India.

The fact that many genera of Tertiary plants are common to North America, the Arctic regions, and Central Europe, is evidence of some former land connection. This connection was probably at the northern ends of the continents, and it allowed the free commingling of the floras of the two bodies of land. When at the close of the Tertiary vast changes took place in the distribution of land and water; and when a wave of extermination swept over the northern and western portion of this continent, the same disaster overtook the forests of Europe. The eastern half of North America and the eastern part of Asia seem to have escaped the effects of the vast change; for in these two regions are still found the remnants of the previous floras. Europe and the Rocky Mountain region suffered from the throes of mountain-making, and the disastrous effects of these convulsions are shown in the extinction of the luxuriant flora and the varied fauna which had previously existed.

Thus it can be seen that our hickory can boast a pedigree which puts to shame the mushroom-growth of modern days; and, while the descent can not be traced in a direct line through all the intermediate stages, we can safely formulate the main facts. The ancestor of both walnut and hickory orignated toward the beginning of the Cretaceous period. The separation of the two occurred toward the end of the same epoch, and they both spread during the highly favorable period of the Tertiary over the whole country and across the Arctic zone into Europe and Asia. The continental condition of Eastern North America and its lack of large, shallow lakes forbade the preservation of such forms as existed there; and the comparatively small portion of our Western region which has been explored has prevented the discovery of many of the species then living there. Yet there can be little doubt but that, if our knowledge of pre-existing species was sufficiently full, we should be able to trace back to some common ancestor all nine of those species of hickory which now live in our country, and the fruits of some of which annually contribute to the enjoyment of hundreds and thousands of our people.