374 THE POPULAR SCIENCE MONTHLY. 



would retire south or up the mountains of the tropics when the cold 

 moderated and the warm season again commenced. Then another 

 season of glacial cold begins in the north. The plants which had be- 

 fore been left on the mountains would stand a good chance of being 

 driven by the increasing cold to the plains, and still farther south, even 

 perhaps across the equator into the southern hemisphere, and when the 

 cold again decreased would retire to the fastnesses of the mountains. 



If we accept this view of the influence of glacial periods on the 

 vegetable kingdom, we shall see that many of the apparently anoma- 

 lous cases of distribution mentioned in the first part of this article are 

 explained. We can easily see why some of the inhabitants of the tem- 

 perate and arctic zones of the northern are represented in correspond- 

 ing zones in the southern hemisphere ; it is easily explained why iden- 

 tical species of mosses are found in Lapland, on mountains of Jamaica, 

 and the Peak of Teneriffe ; why plants of the Pyrenees are identical 

 with those of the Arctic regions ; why species are found on the White 

 Mountains of New Hampshire and in Greenland, but not in the inter- 

 vening region, and why species are found in northern Europe and 

 America, in Chili and New Zealand. This theory of alternate hot and 

 cold periods is as yet the only one by means of which these cases can 

 be explained. 



Such, then, are some of the natural methods for the distribution of 

 plants ; the air, the water, beasts, birds, and fishes, as we have shown, 

 all perform their several offices ; but there is still another method of 

 transport of which nothing has been said, and this is the part which 

 man plays in the grand work. This is by no means insignificant, and 

 can be shown in many ways. Out of the 2,582 species given in Gray's 

 "Manual of Botany", there are 305 introduced species, and, of these, 

 278, all but 27, were imported from Europe. Nothing shows more strik- 

 ingly man's influence than this fact, which is further corroborated by the 

 assertion that the greater part of the plants naturalized at the Cape of 

 Good Hope and in Australia are of European origin.* With the great 

 increase of facilities for travel, on land and on sea, with the extension 

 of commerce to all quarters of the globe, and with the settlement and 

 consequent clearing off of formerly unoccupied lands, we find both the 

 fauna and flora of many countries greatly modified. There can be no 

 more striking example of this influence of mankind then that shown in 

 the Island of St. Helena. " When St. Helena was discovered, about the 

 year 1506," says Lyell,f " it was entirely covered with forests, the trees 

 drooping over the tremendous precipices that overhang the sea. Now, 

 says Dr. Hooker, all is changed ; fully five sixths of the island is en- 

 tirely barren, and by far the greater part of the vegetation which exists, 

 whether herbs, shrubs, or trees, consists of introduced European, Amer- 

 ican, African, and Australian plants, which propagated themselves 

 with such rapidity that the native plants could not compete with them. 

 * Lyell, "Principles of Geology," vol. ii., p. 402. f Lyell, ibid., vol. ii., p. 457. 



