INTRODUCTION Lfi 



vast continent, "Gondwana Land/' which also included a pari 

 of the antarctic continent. How far these connections wei 

 maintained during the Mesozoic and Tertiary seems to be very 

 uncertain; but some such connections would seem to be nec< - 

 sary to account for the present distribution of these Bub-antarctic 

 plants. 



Existing Factors in Distribution 



Among the factors of prime importance in the distribution of 

 the present floras of the earth, perhaps the first is the relation of 

 the great continental masses to each other, and to the great bodies 



of water surrounding them. 



The proximity of Eurasia and North America, especially in 

 the North Pacific region, is reflected in the evident relationships 

 of the northern floras throughout the northern hemisphere, while 

 in the southern hemisphere the plants of the different continents 

 are much less closely related, as might be expected from the isola- 

 tion of the principal land-masses. 



The oceans are efficient barriers to the migration of all but a 

 small number of plants, and high mountain ranges form obstacles 

 to the passage of plants, as do extensive arid regions such as occur 

 in the centre of most of the continents. While mountain chains 

 act as barriers they also may serve as highways, as for instance 

 in the Sierra Nevada and Rocky Mountains where many arctic 

 and sub-arctic species have migrated southward far beyond their 



original homes. 



Many plants, however, have developed special contrivani 

 for distributing their seeds and spores, which enable them to 

 spread rapidly, and overcome to some degree the barriers of Sea 

 and mountain. 



Many of the lower plants, including certain ferns, produce many 

 very light spores, which retain their vitality for long periods and 

 may be carried great distances by air-currents. Thus ferns are 

 among the earliest plants to invade oceanic islands, where the 

 original vegetation has been destroyed by volcanic action or other- 

 wise. 



The winged fruits and seeds of thistles, dandelions, milkweed, 

 and many other similar plants may be borne by the wind very far 

 from their origin and enable these weeds to spread with great 



