APPENDIX. No. Y. 463 



In Russel's catalogue of the plants of Aleppo two only are noticed : and 

 even in I\I. Desfontaines' Flora Atlantica not more than eighteen species 

 occur, or with relation to the Phaenogamous plants, about one to one hundred. 



The Ferns in the herbarium from Congo, are to the Phaenogamous plants as 

 about one to twenty-six, which agrees nearly with their proportion in Forskal's 

 catalogue of the plants of Arabia, with that of the north coast of New Holland, 

 according to my own observations, and which is probably not very different 

 from their proportion in India. 



In concluding here the subject of the proportional mmibers of the Natural 

 Orders of plants ccmtained in the herbarium from Congo, I may observe, that 

 the ratios I have stated, do not always agree with those given in Baron 

 Humboldt's learned dissertation, so often referred to. I have ventured,, 

 however, to differ frnin that pmincnl naturaUst with the less hesitation, as he 

 has expressed himself dissatisfied with the materials from which his equinoctial 

 proportions are deduced. Whatever may be tlie comparative value of the 

 i'acts on which my own conclusions depend, I certainly do not look upon them 

 as completely satisfactory in anv case. And it appears to me evident, that 

 with respect to several of the more extensive natural orders, other circum- 

 stances besides merely the degrees of latitude and even the mean temperature 

 must be taken into account in determining their relative numbers. To ariive at 

 satisfactory conclusions in such cases, it is necessary to begin by ascertaining 

 the geographical distribution of genera, a subject, the careful investigation of 

 which may likewise often lead to important improvements in the estabhshment 

 or sub-divisions of these groups themselves, and assist in deciding from what 

 regions certain species, now generally diffused, may have originally proceeded. 



To the foregoing obsenations on the principal Natural Orders of Plants 

 from the banks of the Congo, a few remarks may be added on such families 

 as are general in equinoctial countries, but which are not contained in the 

 collection. 



which may be called Adiantum africanum, 1 have collected specimens in Madeira, and- 

 have seen others from Tcneriffe, St. Ja^o, Mauritius or Isle de Bourbon, and Abyssinia. 

 Adiantum africanum has also been confounded with A. tenerum of Jamaica, and other 

 West India Islands, and the latter with A. capillm venerit, which has in consequence been ■ 

 supposed common to both hemispheres, to the old and new continent, and to the torrid 

 and temperate zones. 



