-iO 



GEOGRAPHICAL DISTRIBUTION. 



The present distribution of the Arctic-Alpine flora is usually explained on an hyp 



thesis of alternating migrations. 1 This hyphothesis may be summarised as follows 



A circumpolar flora initially uniform in character and, during the warmer period 

 antecedent to one or other of the more general glacial periods, compact in distribution, 

 commenced to migrate southwards under the influence of the gradual decrease of tem- 

 perature that preceded the glacial period proper. During this advance new forms that 

 appeared did so under conditions that were uniform for the migrating flora. At the same 

 time the progressive enlargement of the area subject to these conditions mitigated the 

 severity of the struggle for existence between rival forms and diminished the swamping 

 effect of the presence alongside of them of the original types. A large proportion of the 

 forms were thus enabled to survive. Here and there accident determined the detention of 

 stragglers, while occasionally a particular meridian of migration ended in a cul-de-sac. In 

 these cases necessity compelled the development of a power of accommodating themselves 

 to circumstances which proved sufficient in a few forms to ensure their survival. Wide seas 

 and lofty mountain chains on whose summits a rigorous climate had already become 

 established formed natural obstacles to the advance, and where these were encountered the 

 advance was checked or deflected. Elsewhere the line of furthest advance formed a 

 continuous zone of distribution. The transition between congeneric forms in a given 



section of this zone was a serial one and represented the evolution of the genus along 

 one meridian of migration ; the transition between forms of successive adjacent meridian- 

 sections was in a closed curve. The natural conditions uniform for the invading flora 

 were progressively unfavourable for the displaced one, and few of the forms characteristic 

 of the latter were able to accommodate themselves to adverse circumstances and at the 

 same time to hold their own against members of an invading flora for which these 



conditions were suited. 



When the advance ceased there ensued a period during which the natural conditions 

 were comparatively stable. The struggle for existence within the invading flora was 

 therefore intensified, since the number of forms continued to increase while the area they 

 inhabited ceased to enlarge. Internal pressure tended to make the invading flora extend 



to localities less favourable for its existence. But in these localities the conditions were 

 favourable for the displaced flora, and here it was able not only to survive, but to resist 

 invasion. Hard pressed from within, successfully opposed from without, many forms 

 gave way in the struggle, and as here one, there another, kind of variation proved more 

 adapted to local conditions, some local colouring became imparted to different districts of 

 the zone in which the Arctic-Alpine flora now lived. 



This period of stability came also to an end. With the gradual increase of tem- 

 perature that marked the disappearance of the ice, the invading flora began to migrate 

 northwards. Like the advance, the retreat was a meridional one moditied by local 

 conditions. Intervening seas checked or deflected it as they did the advance. Mountain 

 ranges, however, played a different part. In place of turning aside the retreating flora 

 they largely attracted it, offering their flanks as routes of safe retreat and their summits 

 as asylums where retreat was altogether cut off. But even with this the area available 

 for the Arctic- Alpine flora was steadily curtailed, the struggle for existence was intensified, 

 the tendency to vary, especially in districts where retreat was delayed, was greatly 

 stimulated, the variations deviated more and more from the original type, while for 

 reasons that are not yet demonstrable, now more, now fewer, forms survived. 



This theory was Erst advanced by Professor E. Forbes (Brit Assoc. Report, 1845, and Mem. Geol. Survey i, 336-432 

 but was greatly extended and elaborated by the late Mr. Darwin {Origin of Species, chap, xi [edit. 6, chap, xii]), and 

 illy applied by Sir J. D. Hooker (Trans. Linn Soc. xxiii. 263). 



