202 STUDIES, SCIENTIFIC AND SOCIAL chap. 



become common weeds, will be left out of consideration 

 altogether. We have to do only with the condition of the 

 vegetation brought about by nature, undisturbed by the 

 effects which have recently been produced by man. 



There are two separate phenomena by which we may 

 estimate the relations of the floras of two countries, both 

 of which are important factors in the comparison — the 

 absence from one country of whole groups of plants which 

 are both common and widespread in the other, and the 

 presence of new types entirely unknown in the other. It 

 is usual to lay much more stress on the latter phenomenon, 

 because the former occurs when there is no essential 

 difference between the floras, the one having been recently 

 derived from the other. Thus, many species, and even 

 genera, of West European plants are absent from Britain, 

 but this does not lead us to consider the British flora 

 as being essentially different from that of Europe, the 

 deficiencies being plainly attributable to the smaller area, 

 the limited range of climate, the recent glacial epoch, and 

 other such causes. But, when the country in which the 

 deficiency occurs is fairly comparable with the other in all 

 these respects, the cause of the phenomenon is evidently 

 a deep-seated one, and must be held to show a funda- 

 mental diversity in their floras. 



There are, of course, in every extensive flora such as 

 that of North America a considerable number of almost 

 cosmopolitan groups or species, and many others which 

 are found in all temperate regions. Thus, no less than 115 

 European genera and 58 European species are found at the 

 antipodes in New Zealand, and many others in Australia 

 and South temperate America. Among these are such 

 familiar plants as buttercups, anemones, poppies, violets, 

 St. John's worts, gentians, forget-me-nots, many genera of 

 cresses and other crucifers, mint, skull-cap, loose-strife, sea^ 

 lavender, and many others ; and there are also in the same 

 remote countries such common English species as the lady's- 

 smock {Cardaminc ]:jTatcnsis), chickweed {Stdlaria media), 

 the cut-leaved geranium {Geranium disscetum), the silver- 

 weed {Potentilla anserina), the common bind-weed {Galy- 

 stegia sepium), and scores of others, all considered to be 



