210 Part UI. Chapter 3. 
western Virginia, northern Georgia, Alabama, Mississippi. A study of these 
maps reveals an important fact, that the spread of the species from this com- 
mon center has been in a series of more or less concentric waves. The map 
accompanying this book, shows the northern limit of many of the most im- 
portant forest trees. The lines may be taken to represent the several distri- 
butional advances of trees from the south. Approximately the trees invaded 
the northern part of the continent in the order indicated by their present 
relative distribution. (See colored map of North America.) Those farthest 
north entered the glaciated country first, either singly, as a species, or by 
mass invasion of associated species and those farthest south advanced much 
more tardily. The order of the invasion depending upon the adaptability of the 
species and the mode of seed distribution was, therefore relatively, as follows: 
Wind Carried Seeds. 
ı. Picea alba (= P. canadensis), 6. Betula papyrifera. 
farthest north. 7. Abies balsamea. 
2. Picea nigra (= P. mariana), 8. Pinus strobus. 
farthest north. 9. Thuja occidentalis. 
3. Larix americana (= P. laricina). 10. Ulmus americana. 
4. Populus. balsamifera. | ır. Acer saccharum. 
5. Populus tremuloides. ı2. Tsuga canadensis. 
Animal Carried Seeds. 
13. Quercus rubra. | 16. Castanea americana (= dentata). 
ı4. Fagus americana. | 17. Juglans nigra. 
15. Quercus alba. | 
a 
The species most successfully provided with means of distribution and most 
easily adjustable extended farthest from the original center after the glacial 
period, which circumscribed the area of the original extended forest of the 
Tertiary period. The map shows that the outer confines of any particular 
genus is usually occupied, by a single species. Nearer the center, two species 
are found; still nearer, if the genus is a large one, three and still nearer, four, 
etc. The position of the various shades of green on the maps suggests the 
eircles of impulse produced when a stone is thrown into water. Theoretically 
these waves spread in all directions, unless they meet with obstacles, when 
they are deflected. Similarly the maps suggest a series of distributional im- 
pulses, by which the various species of oaks, ashes, hickories, and chestnuts 
moved out from a parent forest of great density into the territory left bare 
by the retreat of the great ice sheet ‘), Clearly, therefore, the deciduous forest 
of eastern North America has been derived from that forest which reaches its 
greatest development in the mountainous region of western North Carolina. 
1) HARSHBERGER, J. W.: An ecologie Study of the Flora of mountainous North Caroline. 
Botanical Gazette XXXVI: 241—258, 368—383. Oct. and Nov. 1903. 
