248 



of the influences of climate and temperature ; to the the latter, the 

 more curious class which relates to the substitution, in one part of the 

 globe, of certain representatives of species not found there, but which 

 abound in other localities, possessing perhaps the same climatal con- 

 ditions as those from which the represented species are excluded. 

 Take, as a case in point, the Ericaceae and their allies. 



" From the southern point of Africa to the North Cape in Mageroe, 

 the heaths extend throughout the Old World, merely leaping over the 

 proper tropical regions. With the same latitudes, the same climate, 

 and similar conditions of soil, we find not a single species of true 

 heath in all America. Other allied plants replace them, plants which 

 at least belong to the same family (the Ericaceae) ; but if we go to 

 Australia, we find under corresponding conditions, not one Ericace- 

 ous plant, but in their place appears an allied, but wholly peculiar 

 family of plants, the Epacris tribe." — p. 240. 



Then again the leafless fleshy Euphorbias of the Old World are re- 

 presented, in form at least, by the Cactaceae of the New ; and yet the 

 Cactaceae, though originally strictly limited to the former, are no 

 sooner introduced to many parts of the latter, than they become per- 

 fectly naturalized, a proof that mere climate and soil have nothing to 

 do with their original location. What, then, is the influential agent? 

 In the inquiry 



"Two essentially different points have to be distinguished. The 

 heath plants occur on dry, sunny, sandy plains ; they extend from the 

 Cape of Good Hope, through Africa, Europe, and Northern Asia, to 

 the extreme limits of vegetation in Scandinavia and Siberia ; these 

 plants are distributed in this great region in such a manner that South 

 Africa has innumerable distinct species, of which, however, never 

 more than a few individuals grow side by side, that then, towards the 

 north, the number of species suddenly diminishes in an important de- 

 gree while the number of individuals increases, till at last, in the north 

 of Europe, a single species, the common heather (Calluna vulgaris), 

 overspreads whole countries in millions of single individuals. In the 

 first place, we readily see that only the first determination, that of the 

 occurrence, relates necessarily to each individual ; while, on the con- 

 trary, the range of extension, and the mode of distribution, indicate 

 causes which have scarcely any importance in reference to the single 

 individual, but very great in relation to the larger groups of plants, 

 which we call species, genus, tribe, &c. From this it follows, that the 

 former only, the occurrence of plants, is related wholly, while the 

 other two are related but partly, to conditions explicable by physical 



