MERIDIANS OP DISTRIBtTIOX. 



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r 



from America sufficiently explains their absence from Labrador and Greenland, the arctic 

 districts into which its meridian of distribution passes. If, on the other hand, we suppose 

 that the character first made its appearance in the Japan-Himalaya meridian, throughout 

 which the number of opposite-leaved species almost equals the number of alternated caved 

 ones, we explain at once the fact that we meet in the two arctic districts— Kan ist chat ka 

 and Alaska— into which this meridian passes, all the four verticillate circumpolar species, 

 as well as the fact that two of these (P. Memicsii and P. Chammonis) are confined to 

 these districts. Of the others, P. amcena extends westward as far only as the Yenisei. 

 P. verticillata extends westward into Lapland. It is probable that the character \v;« 

 evolved during some period of southward migration anterior to the most recent on , and 

 that P. verticillata and P. amoena, or some closely representative species, reached tho 

 circumpolar province during the northward migration that followed. The intra-arctic 

 cleavage between Alaska and Greenland is, from the evidence adduced by Sir J. D. Hooker, 1 

 of a high antiquity, so that only a westward zonal distribution was open to them. But 

 they had not extended equally far when the succeeding southward migration commenced, 

 hence it happens that while P. verticillata and P. amcena 1 have both migrated along the 

 Siberian meridian of distribution, P. verticillata alone, having by the same time extended 

 sufficiently far west, has migrated along the European meridian of distribution also. Why 

 P. amoena should have proved unstable in its meridian of distribution (l\ violascem is 

 evidently an immediate derivate), while there and in Europe P. verticillata has remained 

 unchanged, also why in the Himalo-Chinese meridian P. verticillata itself has been unstable 

 (P. refracta and P. ssetschuanica being clearly immediately derived forms), are problems 

 which it would be premature in the present state of our knowledge to attack. But the 



facts are nevertheless very striking. 



The considerable number of opposite-leaved forms on the Siberio-Caucasus meridian is 

 to some extent capable of explanation on the assumption that the character was evolved in 

 the adjacent Himalo-Chinese meridian without necessarily postulating a retreat for all the 

 characteristic forms as far as the present Circumpolar province. For during periods of 

 stability marking the line of furthest advance, opposite-leaved species extended zonally 

 westward to what is now the Caucasus province. Eastward they either did not extend 

 or, if they did, the tracts into which they passed are now submerged. Westward beyond 

 the Siberio-Caucasus meridian their extension was checked by an ancient A r al o - Casp i a 

 northward projection of that Mediterranean Ocean which in early Tertiary times separated 

 part of India from the rest of Asia, and extended westward to Europe. During periods 

 of retreat, too, many forms were attracted from the Himalo-Chinese meridian to the 

 southern and eastern flanks of the extended mountain system, which in times of advance 

 had delimited the Siberian and the Chinese meridians, but had now, owing to changed 

 physical conditions, become the backbone of the former. As time went on these forms 

 overflowed, through its passes, what had been at first their barrier wall, and commingled 

 with forms that had been similarly attracted to the northern and western flanks of the 



same system from the Siberian plains. 



The northern shores of the Early Tertiary Mediterranean Ocean prevented the exten 

 sion of the genus southward beyond the present limits of the European and the Caucasian 



* 



i 



1 Trans. Linn. Soc. vol. xxiii, p. 252. 



nmelia), along the Siberian 

 uau ; P. amanto is confined 



and 





Ann. Eoy. Bot. Gard. Calcutta, Vol. III. OT N 





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