1915-16.] BOTANICAL SOCIETY OF EDINBURGH 37 



and the directions in wliicli the plant streams have 

 migrated ? I must confess that my botanical and geo- 

 graphical knowledge is far from equal to this task ; but, 

 as already stated, some advance in knowledge may be 

 made by working with the weapon at my disposal, and 

 I feel sure that botanists wdio have gone properly into the 

 subject will be able to furnish evidence sufficient either to 

 supplement or to destroy the ideas here put forward. 



The Theory 'Tested. 



The best \vay to set about the task is to ask what might 

 be expected to result, so far as the distribution of plants 

 is concerned, from the above suppositions, namely : (i) a 

 continuous Sino-Himalayan range stretching eastwards into 

 China ; (ii) a subsequent breach formed, and a ruckling in 

 the gap such that the broken halves of the original range 

 are completely severed, while the western half remains 

 more or less in communication with the new parallel 

 ranges at right angles ; (iii) immense erosion finally 

 separating the parallel ranges from one another, so that 

 the distribution of species on them is discontinuous. At the 

 same time new rivers are formed and old ones rejuvenated, 

 so that, cutting their way back, they are able to capture 

 rivers belonging to the new system of drainage. A mountain 

 range of not too great altitude is an ideal route for the 

 migration of plants, especially above the tree limit. There 

 is, at least in the earlier days of its uplift, nothing to 

 prevent a plant furnished with the most elementary 

 means of -seed-dispersal spreading from end to end, as 

 conditions in the alpine belt at least are likely to be very 

 uniform throughout the length of the range. Consequently, 

 there is not much room for variation in the flora on this 

 account. Even though the rainfall may be considerably 

 greater on one range than on the next, and on one part 

 of a range as compared with another part, the atmo- 

 sphere is often so full of moisture, even when it is not 

 actually raining, that what with the blankets of cloud 

 hanging over the vegetation and the dew deposited owing 

 to radiation from the bare rock, there is little diflerence 

 in the alpine flora as the snow-line is approached ; melting 



