agreed at the present time. A large number of the thousand 

 different plants that are found in the high Alps reappear in 

 distant mountains and the Arctic regions, though absent in 

 the intervening country. Thus the late John Ball calculated 

 that 17 per cent, of the Alpine species were met with in 

 the Arctic regions, and 25 per cent, in the Altai Mountains 

 of Northern Asia. That this is not due to the conveyance 

 of seeds from place to place by the agency of wind or birds 

 is shown by the fact that the resemblance of the plants of 

 different mountain chains to one another is in no way 

 proportional to their geographical proximity, even when their 

 climate is similar. The true explanation is probably to be 

 sought in the Glacial Period, in which the greater part of 

 Europe was covered by ice, so that the climate of the plains 

 was very similar to that of the high Alps and Arctic regions 

 at the present time. At this period the flora of the extreme 

 north was able to mingle with that of the mountain regions. 

 When the climate of the intervening districts becamfe once 

 more less severe, the competition with the present lowland 

 flora became so keen that the Alpine-Arctic plants retreated 

 to high altitudes or the far north. It may be that the high 

 mountain regions have formed centres for the evolution of 

 new species, as Mr John Ball believed, but there is but little 

 direct evidence for this view. 



Nearly every visitor to Switzerland who regards its rich 

 flora in any way worthy of notice is desirous to obtain some 

 permanent record of the pleasing impressions obtained. Either 



xxii 



