CHAP. VII. BOTANICAL GEOGRAPHY. 309 



(monocarpeans') is greatest in temperate regions, and 

 diminishes both towards the equator and poles. 



Many local circumstances produce remarkable mo- 

 difications in the relative proportions between the 

 species of different classes and orders, in regions under 

 the same parallels of latitude. Thus for instance, ceteris 

 paribus, the cryptogamic tribes flourish most in moist 

 regions. The places best adapted to the growth of 

 ferns are the ( islands in tropical climates, in some of 

 which, as in St. Helena, one half the flora is composed 

 of them. It is remarkable that in this respect, and as 

 regards the existence of arborescent species in this 

 order, the ancient flora of our coal-fields, appears to 

 approximate very closely to that of islands situate in 

 the midst of an extended ocean and in low latitudes. 

 The same causes which appear favourable to the in- 

 crease of cryptogamic species, seem also to produce 

 a diminution in the proportions which dicotyledons 

 bear to monocotyledons. Other relations of consi- 

 derable interest have been pointed out between the 

 species of different orders, occurring in different re- 

 gions ; but we cannot enter into the minutiae of their 

 details, our object being rather to present the reader 

 with the principles on which such investigations depend, 

 than to acquaint him with the partial results which 

 have hitherto been deduced from them ; several of 

 which must doubtless be greatly modified hereafter, 

 considering the little knowledge we at present possess 

 of the floras of many parts of the world. 



Th,e following table exhibits a few of those results 

 which appear to have been most satisfactorily esta- 

 blished. It gives the relative proportion which ten 

 well-defined orders, or families of plants, bear to the 

 whole of the phanerogamic tribes in the torrid, tem- 

 perate, and frigid zones respectively, and shows us in 

 which they occur in the greatest relative abundance, 

 decreasing as we recede from that zone towards the 

 others. 



x 3 



