298 PHYSIOGNOMY OF PLANTS. 



We may, if we please, pass from the consideration of species to 

 that of divisions formed in the natural system of botany according 

 to an ideal series of abstractions, and direct our attention to Genera, 

 to Families, and even to the still higher, i. e. more comprehensive, 

 Classes. There are some genera, and even some entire families, 

 which belong exclusively to particular zones of the Earth's surface ; 

 and this not only because they can only flourish under a particular 

 combination of climatic conditions, but also because both the local- 

 ities in which they originated, and their migrations, have been 

 limited. It is otherwise with the greater number of genera and of 

 families, which have their representatives in all regions of the globe, 

 and at all latitudes of elevation. The earliest investigations into 

 the distribution of vegetable forms related solely to genera ; we find 

 them in a valuable work of Treviranus, in his Biology (bd. ii. s. 47, 

 63, 83, and 129). This method is, however, less fitted to afford 

 general results than that which compares either the number of spe- 

 cies of each family, or the great leading divisions (of Acotyledons, 

 Monocotyledons, and Dicotyledons) with the sum of all the phanero- 

 gamse. "VVe find that in the cold zones the variety of forms does 

 not decrease so much if estimated by genera as if estimated by spe- 

 cies ; in other words, we find relatively more genera and fewer spe- 

 cies. (Decandolle, The"orie el&nentaire de la Botanique, p. 190; 

 Humboldt, Nova genera et species Plantarum, t. i. pp. xvii. and 1.) 

 It is almost the same in the case of high mountains whose summits 

 support single members of a large number of genera, which we should 

 have been d priori inclined to regard as belonging exclusively to the 

 vegetation of the plains. 



I have thought it desirable to indicate the different points of view 

 from which the laws of the geographical distribution of plants may 

 be considered. It is by confounding these different points of view 

 that apparent contradictions are found, which are unjustly attributed 

 to uncertainties of observation. (Jahrbiicher der Gewachskunde, 

 bd. i. Berlin, 1818, s. 18, 21, 30.) When such expressions as the 

 following are made use of " This form, or this family, diminishes 

 as the cold zones are approached; it has its true home in such or 

 such a latitude ; it is a southern form ; it predominates in the 

 temperate zone;" care should always be taken to state expressly 



