TRANSACTIONS OF SECTION E. 



731 



dwells at length on the value of numerical data, and explains his ' Arithmeticse 

 botanices ' which consists in determining the proportion which the species of certain 

 large families or groups of families bear to the whole number of species composing the 

 ■floras in advancing from the Equator to the Poles, and in ascending mountains. Some 

 kinds of plants, he says, increase in numbers relatively to others in proceeding 

 from the Equator to the Poles, as ferns, grasses, amentiferous trees, &c. ; others 

 decrease, as Rubiaceae, Malvaceae, Composite, &c. ; whilst others still, as Labiatse, 

 Cruciferse, &c., find their maximum in temperate regions, and decrease in both 

 directions. He adds that it is only by accurately measuring this decrease or 

 increase that laws can be established, when it is found that these present constant 

 relations to parallels of temperature.' Furthermore, he says that in many cases 

 the whole number of plants contained in any given region of the globe may be 

 approximately determined by ascertaining the number of species of such families. 



The importance of this method of analysing the vegetation of a country _ in 

 researches in geographical botany is obvious, for it affords the most instructive 

 method of setting forth the relations that exist between a flora and its geographical 

 position and climatai conditions. 



Humboldt's labours on the laws of distribution were not limited to floras, they 

 included man and the lower animals, cultivated and domesticated, as well as 

 native ; they may not be works of the greatest originality, but they show remark- 

 able powers of observation and reflection, astonishing industry, conscientious 

 •exactitude in the collection of data, and sagacity in the use of them: he is indis- 

 putably the founder of this department of geographical science. 



No material advance was made towards improving the laws of geographical 

 distribution - so long as it was believed that the continents and oceans had ex- 

 perienced no great changes of surface or of climate since the introduction of the 

 existing assemblages of animals and plants. This belief in the comparative sta- 

 bility of the surface was first dispersed by Lyell, who showed that a fauna may be 

 older than the land it inhabits. To this conclusion he was led by the study of 

 the recent and later tertiary molluscs of Sicily, which he found had migi-ated into 

 that land before its separation from the continent of Italy ; just, he adds, as the 

 plants and animals of the Phlegraean fields had colonised Monte Nuovo since that 

 mountain was thrown up in the sixteenth century ; whence, he goes on to say, we 

 are brought to admit the curious result, that the fauna and flora of Val de Noto, 

 and of some other mountain regions of Italy, are of higher antiquity than the 

 country itself, having not only flourished before the lands were raised from the 

 deep, but even before they were deposited beneath the waters.^ The same idea 

 occurred to Darwin who, alluding to the very few species of living quadrupeds 

 •which are altogether terrestrial in habit, that are common to Asia and America, 

 and to these few being confined to the extreme frozen regions of the North, adds, 

 ' We may safely look at this quarter (Behring's Straits), as the line of communica- 

 tion (now interrupted by the steady progress of geological change), by which the 

 elephant, the ox, and the horse entered America, and peopled its wide extent.' * 



The belief in the stability of climatai conditions during the lifetime of the 

 existing assemblages of animals and plants was also dispelled by the discovery, 

 throughout the northern temperate regions of the old and new worlds, of Arctic 



' Humboldt's Isothermal lines and Laws cf geographical distribution are obviously 

 the twin results of the same researches, one physical the other biological. 



- I do not hereby imply that no progress was made in the knowledge of the 

 facts of distribution, for over and above many treatises on the distribution of 

 the plants of local floras, there appeared, in 3 816, Schouw's Dissertatio de sedibus 

 j)lantarum orifjinariis ; which was followed in 1822 by his excellent Grnndtrack til 

 el almendelig Planic- Geographic, of which the German edition is entitled, Grundzvge 

 einer allgemeincr Pflanzengeograplde. 



' Principles of Geology, ed. 3, vol. iii. p. 376, 1834. 



♦ Journal of Besearchei in Geologrj and Natwral History, ^•e., p. 161, 1839. 



