302 BRITISH PLANTS 



Nevertheless, the law, unfortunately, cannot be uni- 

 versally applied. Some qualities blend in the hybrids, 

 and there the law, as at present expressed, fails. Sooner 

 or later it may be shown that Mendel's Law is only a 

 special case of a wider generalization. When this is 

 found the whole problem of hereditary transmission is 

 within sight of solution. 



APPENDIX in. 



Botanical Provinces. — At the present time the flora 

 of the world is divided into a number of separate Botan- 

 ical Provinces or Floral Regions, just as the world of 

 living men is divided ethnologically into distinct race- 

 divisions. Each Botanical Province is distinguished by 

 the presence of certain groups of plants, species, genera 

 or natural orders, which differ as we pass from one pro- 

 vince to another, although the conditions of soil and 

 climate may be similar. Different authorities divide 

 up the earth differently, but the classification usually 

 followed is that of Drude, who distinguishes fourteen 

 Floral Regions. Europe is divided into only two Botan- 

 ical Provinces : (1) the Northern, and (2) the Southern or 

 Mediterranean Province, the boundary being formed by 

 the Pyrenees and the Alps. The Northern Province 

 extends from Europe through Siberia to the Pacific and 

 also includes the northern portion of the American conti- 

 nent. The uniformity of the flora throughout the 

 Northern Temperate regions of the world suggests, by 

 itself, a former union between the Old and the New 

 Worlds, either across the Northern Atlantic or Behring's 

 Strait or both, an inference which is supported by several 

 other considerations. 



This lack of uniformity in the present flora of the world 

 was not always so. As far as we know it dates oidy from 

 the Glacial Period. Before then there seems to have 

 prevailed throughout geological history, with one excep- 

 tion, a great uniformity of climate and consequently of 

 flora also. This one exception was the Permo-Carboni- 

 ferous Period, when there were two marked Botanical 

 Provinces in the world, the Northern Province, north of 

 the Equator, and the Southern or Gondwanaland Province 



