OX THE MODES OF DISTRIBUTIOX OF PLAXTS. 373 



locusts' dung received from South Africa, Mr. Darwin extracted and 

 raised seeds of seven grass-plants, which belonged to two species of 

 two genera.* These locusts are sometimes found as far as three hun- 

 dred and seventy miles from land ; and an account is given of a cloud 

 which hovered round the Island of Madeira for three days, and then 

 disappeared without alighting. Such a cloud as this would undoubt- 

 edly be capable of introducing the seeds of foreign plants into insu- 

 lated localities. The immense number of grasshoppers which have 

 devastated the plains of Kansas and Nebraska would in tho same way 

 be the means of introducing seeds of foreign plants. 



There is still another method which has been at times used by Na- 

 ture for the distribution of plants, and that is by means of the alter- 

 nation of hot and cold epochs, commonly known as glacial periods. 

 Now, it has been demonstrated beyond all doubt that at one period of 

 the earth's history the Arctic regions were much warmer than they are 

 at present ; this is proved by the occurrence in the geological forma- 

 tions of these high northern latitudes of plants in a fossilized state, 

 which were utterly incapable of existing in any latitude where the cli- 

 mate was colder than it is now in our temperate regions. Reasoning 

 from analogy and our knowledge of the present distribution of Arctic 

 plants, it would not be improbable that the plants inhabiting the lands 

 of the pole were the same on all longitudes of the Arctic Circle. Let 

 us, then, suppose the glacial period to commence in these warm lands. 

 Each plant, following to a greater or less degree the longitudinal line 

 on which it grew, would be slowly but steadily driven by the increas- 

 ing cold to take refuge in warmer and more southern stations. The 

 cold, in the course of years following them slowly up, would compel 

 them to keep continuing their journey southward until such time as 

 the maximum of cold had been reached. Then, if, as it is reasonable to 

 suppose, many of these plants had migrated on the longitudinal line 

 upon which they had lived directly southward, we would find that the 

 plants, which at the Arctic Circle, or beyond, had lived in close prox- 

 imity to each other, would be separated when they reached the tem- 

 perate zone by hundreds of miles. 



In the general journey southward, the plants of the mountains 

 would descend to the plains and mingle with those of the far north. 

 Then the climate commences to moderate ; and, as the mountains of 

 ice and snow retire to their original homes in the north, many of the 

 plants would keep company with the cold and return, but many others, 

 encountering mountains in their paths, would find the climate cold 

 enough for their growth, and would be left there in isolation while 

 their nearest relatives would be separated from them by hundreds of 

 miles of country. 



Now would come the cold period of the southern hemisphere and 

 drive the plants inhabiting the country there northward, and these again 



* " Origin of Species," p. 327 



