ON THE GEOGRAPHY OF PLANTS. 



493 



those families which abound in alpine species ; as, for instance, Gramineae and 

 Composite. At 6000 feet of elevation, the mean temperature of the air, on 

 the back of the equatorial Andes, is 62° 6', which is equal to that of July at 

 Paris. Although, upon the table-land of the Cordilleras, we find the same 

 annual temperature as in high latitudes, yet it is not right to generalise too 

 much, such analogies between the temperate climates of equatorial mountains 

 and low stations in the circumpolar zone. These analogies are not so great as 

 is supposed ; they are much influenced by the partial distribution of heat in 

 different seasons of the year. The quotient does not regularly change, in 

 rising from the plains into the mountains, in the same manner as it does in 

 approaching the pole ; as happens with monocotyledones in general, ferns, 

 and Composita?. 



We may, moreover, remark, that the development of the vegetation of 

 different families depends neither upon geographical nor isothermal latitude 

 alone; but that, on the contrary, the quotients are not in accordance on 

 the same isothermal line of the temperate zone in the plains of America 

 and of the old world. Under the tropics, there is a remarkable difference 

 between America, India, and the western side of Africa. The distribution of 

 organised Jbeings over the surface of the globe depends not only upon very 

 complicated conditions of climate, but also upon geological causes, the nature 

 of which is wholly unknown, but which are connected with the original state 

 of our planet. In the equinoctial zone of Africa, palms are not very numerous, 

 if compared with the much greater number in South America. Differences 

 such as these, far from turning us from a search after the laws of nature, 

 should, on the contrary, excite us to contemplate those laws in their most 

 complicated forms. Lines of equal heat do not follow the parallel of the 

 equator ; they have convex and concave summits, which are distributed very 

 regularly over the globe, and form different systems along the eastern and 

 western sides of the two worlds, in the centre of continents, and in the vicinity 

 of oceans. It is probable that, when the globe shall have been more correctly 

 examined, it will be found that the lines of maxima of grouping will be 

 isothermal lines. If we divide the -globe into lines of longitude, and compare 

 the numerical proportions of those lines under similar isothermal latitudes, the 

 existence of different systems of grouping will at once be evident. From such 

 systems can be distinguished, even in the present imperfect state of our 

 knowledge, those of the new world, of western Africa, of India, and of New 

 Holland. As we find that, notwithstanding the regular increase of heat from 

 the equator to the poles, the maximum of heat is not always identical in 

 different countries in different degrees of longitude; so there exist places 

 where certain families attain' a^'greater degree of development than elsewhere ; 

 as is the case with the Composita? in the temperate region of North America, 

 and especially at the southern extremity of Africa. 



