DR. SCHLEIDEX'S THEORY OF AGRICULTURE. 33 



desired characters, and if tlie experiment be continued through a 

 series of years, the variety at last will constantly produce plants 

 like itself, and will be then an established subspecies. 



The necessary condition is, that the same circumstances of 

 soil and cultivation should be continued through the whole pro- 

 cess, and it is clear that by inverse treatment a plant may be 

 made to revert to its original type. 



The object tiien of field-culture is not plants as species, but as 

 varieties. And as such they may be arranged in three classes. 

 With the first of these, which regai-ds mere general vigour of 

 growth, as in varieties of real or artificial grasses and forest trees, 

 we have at present little interest. Preternatural development of 

 certain parts and modifications of chemical action are much more 

 to our purpose. The following list comprises the most familiar 

 instances : — 



A. Preternatural development of certain Parts. 



a. Boot . . Turnips. 



b. Stem . . Kohl-rabi, Cauliflower, Flax, and Hemp. 



c. Leaves . Cabbages. 



d. Buds, especially subterranean. Potato. 



e. Fruit . . Apple, &c. 



/. Seed . . Cereals, Pulse, and Oil Plants. 



B. Modifications of Chemical Action with a view to the Multiplication of 

 peculiar Constituents. 



a. Protein . Cereals, Pulse, and Coleworts. 



b. Starch . . Potato, Cereals, Pulse. 



c. Sugar . . Beetroot. 



d. Oil . . . Oil Plants. 



The preternatural development of one part supposes peculiarities 

 of chemical action in the whole. What then are the conditions 

 which can induce such modifications ? 



To this end we must compare the circumstances attendant on 

 natural and artificial growth, which will throw much interesting 

 light on the nutrition of plants, and some other important points 

 of their culture, as for instance the nature of manures. 



Now there are soils extremely rich in organic substances, as 

 peat bogs or moors. They support a very poor vegetation ; poor 

 in the number of species, their uniformity of habit, and their all 

 but utter worthlessness. There are soils again rich in organic 

 substances, consisting to the depth of 100 feet or more of humus, 

 which support the most luxuriant vegetation, viz., the tropical 

 virgin forests : their richness in species, their variety of forms, and 

 their great utility form a striking contrast to the case just men- 

 tioned. Again, there are soils consisting of lava and volcanic 

 ashes, with scarce a trace of humus, except such as arises from 

 the still undecoraposed remains of the plants which they have 



VOL. VII. ^ 



