GEOGRAPHICAL DISTRIBUTION. 



99 



also they reach their maximum numbers. Mirbel states 

 that, while in Spitzbergen, at latitude 80°, there are only 

 about thirty species, in Madagascar, under the tropic of 

 Capricorn, between latitudes 13° and 24°, there are no less 

 than five thousand, thus showing the numerical relation of 

 species to temperature and other climatal conditions. But 

 the proportion of cryptogamic to phanerogamic vegetation 

 increases on the other hand from the Equator to the Poles ; 

 for, while on equatorial plains it is as one to fifteen, and on 

 equatorial mountains as one to five, in the temperate zone it 

 is as one to two, and in the frigid zone as one to one ; until 

 the vegetation becomes wholly cryptogamic, thus illustrating 

 the law that the simplest plants are most widely diffused 

 over our globe. There is probably a similar, though not so 

 high a proportion, between Lichens and other cryptogams 

 in their longitudinal diffusion, though this has never been 

 accurately determined. In taking a survey of the horizontal 

 range of lichenose vegetation over the world, it is found, on 

 the other hand, that in particular countries or hemispheres 

 certain species, genera, or families have only a limited dif- 

 fusion, or attain only a minimum development. The TJm- 

 bilicarias, which are very common in Arctic regions, are re- 

 presented by a single species in the Antarctic, where they 



