In studying the geographical distribution of forms, we may consider the species, genera, and natural families of plants separately. A single species, especially among social plants, frequently covers an extensive tract of land. Thus we have in the north, Pine or Fir forests, and Heaths (_ericeta_); in Spain, Cistus groves; and in tropical America, collections of one and the same species of Cactus, Croton, Brathys, or Bambusa Guadua. It is interesting to study more closely these relations of individual increase, and of organic development; and here we may inquire, what species produces the greatest number of individuals in one certain zone; or, merely what are the families to which the predominating species belong in different climates. In a very high northern latitude, where the Compositæ and the Ferns stand in the ratios of 1 : 13 and 1 : 25 to the sum of all the phanerogamia (_i. e._, where these ratios are found by dividing the sum total of all phanerogamia by the number of species included in the family of the Compositæ, or in that of the Ferns); one single species of Fern may, however, cover ten times more space than all the species of the Compositæ taken together. In this case the Ferns predominate over the Compositæ by their mass, and by the number of the individuals belonging to the same species of Pteris, or Polypodium; but they will not be found to predominate, if we only compare the number of the different specific forms of the Filices, and of the Compositæ, with the sum total of all Phanerogamia. As, therefore, multiplication of plants does not follow the same laws in all species, and as all do not produce an equal number of individuals, the quotients obtained by dividing the sum of all phanerogamic plants by the species of one family, do not _alone_ determine the leading features impressed on the landscape, or the physiognomy of nature peculiar to different regions of the earth. If the attention of the travelling botanist be arrested by the frequent repetition of the same species, by its mass, and the uniformity of vegetation thus produced, it will be still more forcibly arrested by the infrequency of many other species useful to man. In tropical regions, where the Rubiaceæ, Myrtles, Leguminosæ, or Terebinthaceæ, compose the forests, one is astonished to meet with so few trees of Cinchona, or of certain species of mahogany (_Swietenia_), of Hæmatoxylon, Styrax, or balsamic Myroxylon. I would also here refer to the scanty and detached occurrence of the precious febrifuge-bark trees (species of Cinchona) which I had an opportunity of observing on the declivity of the elevated plains of Bogota and Popayan, and in the neighbourhood of Loxa, in descending towards the unhealthy valley of the Catamayo, and to the river Amazon. The _febrifuge-bark hunters_ (Cazadores de Cascarilla), as those Indians and Mestizoes are called at Loxa, who