78 THE POPULAR SCIENCE MONTHLY. 



America. The lowest series of Tertiary rocks in Europe contains re- 

 mains 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 north- 

 ern 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 Amer- 

 ica 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 dis- 

 covery of many of the species then living there. Yet there can be 

 little doubt but that, if our knowledge of pre-existing species was suffi- 

 ciently 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. 



