ILLUSTRATIONS (13). RATIO OF DISTRIBUTION. 285 



in proportion as it recedes from the equinoctial zone to the 

 north pole. If we find its ratio for the torrid zone (from 0° 

 to 10° of latitude) —, we shall have for the part of the tem- 

 perate zone (lying between 45° and 52°) y 1 ^, and for the frigid 

 zone (between 67° and 70° lat.) only -^j. The direction 

 followed by the great family of the Leguminosae (viz., increase 

 towards the equator) is also that of the Rubiaceae, the Euphor- 

 biaceee, and especially the Malvaceae On the other hand, 

 the Gramineae and the Juncaceac (the latter more than the 

 former), the Ericeae, and Amentaceae, diminish towards the 

 torrid zone. The Compositac, Labiatae, Umbelliferae, and 

 Cruci ferae, diminish from the temperate zone towards the pole 

 and the equator, and the two latter families most rapidly in 

 the direction of the equatorial region ; whilst in the temperate 

 zone the Cruciferae are three times more abundant in Europe 

 than in the United States of North America. In Greenland 

 the Labiatae are reduced to only one species, and the Umbel- 

 liferao to two, while the whole number of the phaneroganiia 

 still amounts, according to Hornemann, to 315 species. 



It must at the same time be observed that the development 

 of plants of different families, and the distribution of their 

 forms, do not depend alone on the geographical, or even on 

 the isothermal latitude ; the quotients not being always equal 

 on one and the same isothermal line in the temperate zone, as 

 for instance in the plains of America and in those of the Old 

 Continent. Within the tropics there is a very marked differ- 

 ence between America, the East Indies, and the western coast 

 of Africa. The distribution of organic beings over the surface 

 of the earth does not depend solely on the great complication 

 of thermic and climatic relations, but also on geological causes 

 which continue almost wholly unknown to us, since they have 

 been produced by the original condition of the earth, and by 

 catastrophes which have not affected all parts of our planet 

 simultaneously. The large pachydermata are no longer found 

 in the New Continent, while they still exist under analogous 

 climates in Asia and Africa. These differences, instead of 

 deterring us from the investigation of the laws of nature, 

 should rather stimulate us to study them in all their intricate 

 modifications. 



The numerical laws of families, the frequently striking 

 agreement between the ratios, where the species constituting 



