DISTRIBUTION AS AFFECTED BY MOISTURE AND HEAT. 663 



the limestone, not merely by their geological characters, but also by 

 the disposition of the arborescent vegetation. On the serpentine, 

 usually pines only grew, and never in thick forest masses, but scat- 

 tered ; whereas the limestone bore thick clustered oaks and a luxu- 

 riant underwood, with now ,and then clumps of lofty pines. In the 

 low countries near the sea, the serpentine was marked by Senecio 

 squalidus, a little Erophila, and Cheilanthes odora ; while on the lime- 

 stone, Acrostichum lanuginosum was a conspicuous fern. Some of the 

 rare alpine plants of Scotland grow on serpentine. A crumbling mica- 

 ceous soil favours the growth of alpine species in Britain. Lichens 

 seem to be often associated with special kinds of rocks. 



Alphonse De CandoUe has recently promulgated the following 

 views in regard to the distribution of plants in connection with heat 

 and moisture : — The present distribution of plants over the globe 

 depends on two principal factors— 1. The phenomena of distribution 

 in other geological epochs than our own. 2. The physical condition, 

 temperature, moisture, etc., now existing. The climate in any region 

 now-a^days may b6^the same as that which prevailed elsewhere at a 

 remote period. The vegetation of the Mediterranean region, as we 

 now know it, once extended as far as Paris, and the present Arctic and 

 Alpine floras were once spread over a large extent of Europe. The 

 flora of the tropics once extended as far as London, as proved by the 

 fossils of the tertiary epoch. De CandoUe establishes five groups of 

 plants according to their physical requirements. 



1. Megatherms (/ieySs, great, Oi^/jbtj, heat), plants requiring a large 

 amount of heat and moisture. Megathermal plants at the present day 

 exist in the tropics, in the plains, and in the hot damp valleys, as far as 

 the 30th parallel. Mean temperature never below 86° F.,* and moisture 

 never deficient. The fossil predecessors of existing Megatherms are much 

 more widely diffused than their descendants. In a very early period 

 they were distributed all over the globe, but since the commencement of 

 the tertiary epoch they have been concentrated more and more in the 

 equatorial regions. The species of this epoch vary in different regions 

 of the globe. They consist mainly of woody plants and climbers, with 

 persistent leaves. Epiphytes abundant in the forests. Such orders as 

 Anonaoeae, Ternstroemiaceae, Gruttiferse, Aristolochiacese, and Piperacese, 

 are amongst the most characteristic plants. 



2. XerophUes, or Xerophilous -plants (f £f o's, dry, cpiXint, I love), a 

 group of plants requiring as much heat, but less moisture. At the pre- 

 sent day such plants thrive in the hot and dry regions between 20th to 

 25th and 30th to 35th degrees of latitude, i.e. in the dry regions extend- 

 ing from California and Texas to Mexico, from Senegal to Arabia and 



* Not a few of the temperatures which follow, as given by De CandoUe and Schouw, 

 now require revision, and we hope that some meteorplogist will soon adequately diseuss 

 the subject from the most recent observations. 



