HYPOTHESIS OF MIGRATION'S. 



II 



Olli' 



The mountains to the north of the zone of furthest advance were not alone in 



attracting the retreating flora. To a less bnt still to an appreciable extent certain forms 

 found an asylum and perhaps a base for further advance on the slopes or summits of 

 raoun tains to the south. 



_ During the retreat the local colouring peculiar to forms of a particular area became 

 imparted to all the districts along the meridian of retreat In this way the circumpolar 

 flora of the second period of stability, while as compact in distribution as tho initial 

 had been, was less uniform in character. On the other hand, mountain systems that had 

 during the advance formed impassable barriers between adjacent meridians ot mi -ration, 

 attracted to their opposite flanks during the retreat the forms they had hitherto kept asunder 

 and permitted through their passes an interchange of influence and of individuals. Individuals 

 agreeing in morphological characters did not all regain the original seat of the flora, but 

 were content or were compelled to remain in one or more of tho Alpine localities m route. 

 Their characters these detained individuals at first transmitted unchanged. But when 

 the period of retreat had ceased, there came a time when natural conditions w« re once 

 more stable. And now even in the moat suitable localities slight modifications ensued, 

 while in localities where retreat had been earliest and most completely cut off, tin's period 

 of stability allowed the retreating flora to develope its latent powers of acconuuodati. »n and 

 permitted the survival of forms that must otherwise have altogether disappeared. But tho 

 somewhat unsuitable conditions of these localities greatly stimulated the tendency to vary, 

 the more suitable varieties surviving and supplanting the original types. Tin's went en 

 till a balance was established and a period (of which the present is probably an example) 

 of approximate equilibrium resulted. This equilibrium was earliest attained in the new 

 circumpolar region, where the natural conditions were those that had characterised the 

 true home of the flora, whatever its geographical position and whether in advance or in 

 retreat. Its greatly circumscribed area caused an enormous reduction in tho number of 

 the forms it contained, but owing to continuity of conditions there were necessarily among 

 the survivors fewer variations from the original types, and these variations were slight. 

 It is thus that in general terms the Circumpolar province is poorer in nuraher of forms, 

 that these forms are generally more archaic in structure and the endemic percentage of 

 its separable forms is lower than in other provinces, while in provinces nearer to the 

 circumpolar, as compared with those more remote, the same things hold good. 



The behaviour of the Equatorial Flora during these alternating migrations has only 

 an indirect bearing on the distribution of the particular genus under review. It need 

 therefore be but briefly alluded to. It was of necessity in all points the opposite of the 

 other— while the Circumpolar flora advanced and was characterised by a zonal distribution, 

 the Equatorial retreated and had a meridional character imparted to its distribution ; 

 while, on the other hand, the Circumpolar flora retreated, the Equatorial advanced, and its 

 mode of distribution became necessarily more zonal. It has been pointed out on a former 

 page that a few forms of Pedicular is are present in the Intermediate Flora. 1 But Sir J. D. 

 Hooker has clearly shown 2 that there are in reality only two Floras. These two are the 

 Circumpolar and the Equatorial; and this Intermediate Flora, which it is convenient 

 to recognise in practice since it simplifies the treatment of the other two, has been composed 

 mainly°of forms left behind by the Equatorial when the Circumpolar was advancing, by 

 the Circumpolar itself when it was retreating. According to the hypothesis of migrations 

 the present period is one of approximate equilibrium subsequent to a Circumpolar retreat; 



1 P. 22. 



■ Godman and Salvia : Biologia Central, Americana , Botany; Commentary on Mr. Hemde/s Introduction, p. Ixv 



Ann. Roy. Bot. Gabd. Calcutta, Vol. III. 



