ORIGIN AND PRESENT DISTRIBUTION OF THE BRITISH FLORA. 417 



THE ORIGIN AND PRESENT DISTRIBUTION OF THE 

 BRITISH FLORA. 



By Rev. Professor G. Henslow, M.A., V.M.H., &c, 



Lecture given October 15, 1907. 



If anyone were to travel from within the Arctic Circle to the Equator he 

 could not fail to be struck, not only with the vast differences in the appear- 

 ances of the floras that would come under his observation, but also with 

 their manifest discrepancy in point of numbers. For while in Lapland 

 not more than 500 species of Phaenogams and 600 Cryptogams would be 

 met with, in a tropical country, as the East Indies, there would probably 

 be found more than 12,000 in all. Moreover, if our traveller proceeded 

 southwards along more than one meridian, he would not invariably meet 

 with the same plants on or about the same latitude ; for if they were sepa- 

 rated by some great physical barrier, as an ocean or range of mountains, 

 representative species would more probably be observed ; i.e. not merely 

 other species of the same genera, but also plants possessing very similar 

 aspects, and often the same habit as others of quite different families. 

 Thus the fleshy- stemmed species of the genus Euphorbia of Africa repre- 

 sent the order Cactaceae of America, and the Epacrideae of Australia 

 represent the Ericaceae of South Africa. 



Again, in ascending a snow-capped mountain in the tropics the same 

 peculiarities would be observed : at the bottom there would be all the 

 luxuriant vegetation of the tropics, and at the limits of vegetation the 

 lichens, like those of Greenland and Iceland ; while the plants of the inter- 

 mediate stages would be identical with, or representative of, the vegetation 

 of the different latitudes between the Equator and the Arctic Circle.'* 



The inference naturally drawn from these facts would be that tem- 

 perature is an important agent in regulating the distribution of plants. 

 Now this to a considerable extent is the case, but there are so many sub- 

 servient and modifying influences, such as moisture, wind, mountain chains, 

 &c, that the same kind of vegetation is by no means necessarily confined 

 to the same degrees of latitude. Thus, e.g., the extension of tropical forms 

 of plants into lower and cooler latitudes in South America and in Natal 

 than in the northern hemisphere is consequent on the greater abun- 

 dance of humidity in the atmosphere. Similarly in the northern and 

 eastern hemispheres the descending of temperate plants — such as species 

 of oak, willow, rose, &c. — on the Cassia mountains to nearly the sea coast 

 in latitude 25° in tropical Asia is due to the same cause ; while, on the 

 other hand, tropical forms of laurel, bamboo, &c. rise to 9,000 feet on 

 extra-tropical Himalayan Mountains. 



* The temperature of the atmosphere about a tropical mountain decreases one 

 degree for every three to four hundred feet in elevation. 



