CHAP. XXIII ARCTIC PLANTS IN NEW ZEALAND 515 



where, the general conditions being more favourable, it 

 was able to establish itself as a permanent member 

 of the flora. Such, generally speaking, was probably the 

 process by which the Scandinavian flora has made its way 

 to the southern hemisphere ; but it could hardly have done 

 so to any important extent without the aid of those power- 

 ful causes explained in our eighth chapter — causes which 

 acted as a constantly recurrent motive-power to produce 

 that '' continuous current of vegetation '' from north to south 

 across the whole width of the tropics referred to by Sir 

 Joseph Hooker. Those causes were, the repeated changes 



and Delphinium Ajacis (the latter only known thirty miles off in corn- 

 fields in Cambridgeshire), with Atriplex patula and A. deltoidea. Gradually 

 the native sand plants — Carices, Grasses, Galium verum, &c., established 

 themselves, and year by year covered more ground till the new introduc- 

 tions almost completely disappeared. The same phenomenon was observed 

 in Cambridgeshire between Chesterton and Newmarket, where, the soil 

 being different, Stellaria media and other annuals appeared in large patches ; 

 but these soon gave way to a permanent vegetation of grasses, composites, 

 &c. , so that in the third year no Stellaria was to be seen. 



5. Mr. T. Kirk (writing in 1878) states that — ''in Auckland, where a 

 dense sward of grass is soon formed, single specimens of the European milk 

 Thistle {Carduus marianus) have been known for the past fifteen years ; 

 but although they seeded freely, the seeds had no opportunity of germinat- 

 ing, so that the thistle did not spread. A remarkable exception to this 

 rule occurred during the formation of the Onehunga railway, where a few 

 seeds fell on disturbed soil, grew up and flowered. The railway works 

 being suspended, the plant increased rapidly, and spread wherever it could 

 find disturbed soil.'* 



Again : — '*The fiddle-dock (Rumex pulcher) occurs in great abundance 

 on the formation of new streets, &c. , but soon becomes comparatively rare. 

 It seems probable that it was one of the earliest plants naturalised here, 

 but that it partially died out, its buried seeds retaining their vitality." 



Medicogo saliva and Apium graveole7is, are also noted as escapes from 

 cultivation which maintain themselves for a time but soon die out.^ 



The preceding examples of the temporary establishment of plants on 

 newly exposed soil, often at considerable distances from the localities they 

 usually inhabit, might, no doubt, by further inquiry be greatly multiplied ; 

 but, unfortunately, the phenomenon has received little attention, and is 

 not even referred to in the elaborate work of De Candolle {Geographic 

 Botanique Raisonnee) in which almost every other aspect of the dispersion 

 and distribution of plants is fully discussed. Enough has been advanced, 

 however, to show that it is of constant occurrence, and from the point of 

 view here advocated it becomes of great importance in explaining the 

 almost world-wide distribution of many common plants of the north 

 temperate zone. 



^ Transactions of the New Zealand Institute, Vol. X. p. 367. 



