CHANGING OF SPECIES. 



13 



But the changes in climate which occur in nature are of quite a 

 different character. 



On the borders of the Sahara, for instance, one finds many species 

 which are trying to colonize a country in which they have no 

 experience. 



What happens under such conditions can be easily seen in, for 

 instance, Tripoli or Egypt. The influence of the arid conditions affects 

 every plant of every species. The death-rate both of species and of 

 individuals in surviving species is enormous, but one can distinctly see 

 the effects of drought reflected in the endeavours of every plant to take 

 on some modification which will enable it to survive. 



I do not wish to say much of my own personal experiences as a" 

 travelling botanist, but the real evidence for the effect of environment 

 in altering species must be sought by field observations extended over 

 many floras. 



It is perhaps in South Africa that the sharp limitations of quite 

 distinct floras according to climate are most clearly seen. The Karoo, 

 the Cape Flats, Table Mountain, South Eastern Cape Colony and Natal 

 show these differences in quite unmistakable manner. But the 

 dependence of species on climate is quite as clearly marked in, for 

 instance, Madagascar; the grassy plains and rocky hills of Imerina are 

 inhabited by a flora which is utterly different from the feverish jungle 

 of the hillsides. In Chile the evidence is equally clear. The gradual 

 changes in flora from the miserable stunted antarctic beeches of the 

 Straits of Magellan to the beautiful temperate forest of Valdivia and the 

 arid thorns and candelabra Cactus of Santiago province are almost as 

 striking as the changes in Cape Colony. 



Of all countries which I have visited this strict dependence of 

 species upon climate is most difficult to realize in the British Islands. 

 Once one has seen other floras, the facts are quite as clear in Britain 

 as anywhere else, but the effects of climate are for the most part much 

 less marked in Britain and indeed in Europe generally. 



Perhaps it is for this reason that the power of adaptation possessed 

 by all plants seems to be so little realized. 



No theoretical difficulties such as those brought forward by Weis- 

 MANN can possibly upset the evidence of the plants themselves. 



Nor am I alone in frankly admitting change of environment as the 

 obvious originator of new species. The principle seems to me to be 

 now either expressly or implicitly admitted in many of these splendid 

 monographs published by the Berlin School and also in the works of 

 other botanists. I need only refer to the works of Henslow, of von 

 Wett STEIN* on seasonal dimorphism, of AdamovicI on the Servian 

 and Balkan floras, of Schulz on Cardamine, and of Engler§ him- 

 self in his papers on Luzula, Arahis, and Cemstium. 



It is exceedingly difficult for anyone who does not live in London to 



* Von Wettstein, Botan. Centralhlatt. Band 81, p. 15. 

 t Adamovic, Engler's Botan. Jahrbuch, 1889-1900 and 1905. 

 t Schulz, Engler's Botan. Jahrbuch, 1903. 

 § Engler, Botan. JahrhucJi, Vol. 36, 1905. 



