34 Dli- SCHLEIDIiN'S THEOKY OF AGIUCULTUKK. 



supportetl, which still are remarkable for their exuberant vegeta- 

 tion, and which have for centuries been equally prolific without 

 any acKlition of manure. Tiie tierra colorada of Cuba, whicli 

 is used for the cultivation of coifee and indigo, is a case in 

 point. 



Again, there are soils void of humus, which support no vege- 

 tation, or one of the very meanest ciiaracter, as for instance the 

 loose sands of the Sahara. The humus then cannot be the 

 cause of fertility, for we find a very poor vegetation on land ex- 

 tremely rich in humus, and a luxuriant growth on that which is 

 destitute of organic constituents. Nor can the presence of water 

 be the only requisite, for if we ascribe the desolation of the 

 desert to the absence of water, we cannot lay the blame on that 

 in the case of peat bogs. If water be added to the loose sand, 

 as in the forests of tlie German marches, we have a uniform and 

 poor vegetation. There remains but one answer : in the soils 

 destitute of humus it must be the mineral inorganic constituents 

 wliich are the cause of luxiu'iance, and to the same cause must 

 be attributed the fertility of the virgin forests of the tropics, in 

 comparison with the barrenness of the peat bogs, though equally 

 rich in organic matter. Fi'om the very mode of formation of 

 peat bogs it follows that all the soluble mineral constituents are 

 washed out and carried off, whereas in the soil of the virgin 

 forests tliey are stored up, insomuch that the continued cultiva- 

 tion of tobacco for some centuries is required to exhaust tiie 

 alkali of the North American woods. 



Now if a comparison be made with our cultivated lands, it is 

 the first superficial glance alone which recognizes any necessary 

 proportion between the fertility of the soil and its richness in 

 hunuis. Analysis shows that no such relation exists, but that the 

 goodness of the soil depends upon its inorganic constituents, so 

 far at least as they are soluble in water, or through continued 

 action of carbonic acid ; and the more abundant and various 

 tliese solutions, the more fruitful is the ground. 



But the matter may be viewed in another light. If we take 

 the Flora of a country, as for instance that of Germany by 

 Koch, which is perhaps the most perfect of its kind, we find 

 plants divisible into two classes, one containing those whose forms 

 are extremely simple and constant, and another those which are 

 rich in remarkable and definite varieties. The latter, with few ex- 

 ceptions, are either Alpine plants, or such as, in our cultivated 

 ground and its neighbourhood, are subjected to the influence of 

 agriculture. This is the most striking when we compare the 

 species of a particular genus with one anotlier, and find that even 

 nearly related species are distinguished in tliis respect according 

 to their natural locality. The one is subject to produce varieties. 



