512 ORGANOLOGY. 



contain a trace of humus, and their only value consists in the quantity 

 of clay they contain. 



There is a prejudice extant among some people, and which unsuspi- 

 ciously lies at the foundation of their view of the nutrition of plants, 

 and that is, that cultivated plants upon prepared soil vegetate under 

 more advantageous circumstances than plants growing wild. The fact 

 seems to be directly the contrary, for in the culture of the land most 

 plants are placed in circumstances so directly opposed to the natural 

 circumstances of their growth, that we have to employ all the art of 

 agriculture for obviating their injurious action. The problem of 

 agriculture consists in covering a given area with plants of the same 

 species. To this end we must first destroy the whole natural vegetation 

 of the soil (uproot the soil), and as much as possible prevent every new 

 growth upon the soil. The mechanical processes necessary for this are 

 attended with injury to the vegetation, which is increased by our yearly 

 carrying away as harvest those products which, with plants growing 

 wild, would remain upon the soil. The working up of the soil, and 

 allowing it to lie fallow, acts injuriously by exposing it to the action of 

 the weather and desiccation from the sun ; at the same time, the decom- 

 position of the substances drawn by the water from the humus is acce- 

 lerated ; and, lastly, the naked and loose earth is exposed to the con- 

 stant washing of the rain. Finally, cultivated plants sustain injury 

 from the fact, that the soil is covered with a species for which it is not 

 naturally adapted, and consequently the produce is never so large as it 

 might be. 



Very different are the results the nearer the culture of the field ap- 

 proaches that of the garden. In this case the cultivated plants have 

 strikingly the advantage of the wild plants of our climate. The garden 

 soil is distinguished by two peculiarities which arise out of the action of 

 excessive manuring. First, it contains all the inorganic elements in the 

 greatest quantity and the most favourable form that is combined with 

 easily decomposable organic substances. Secondly, it has, on account of 

 the quantity of humus it contains, the capacity of supplying the growing 

 plants with the organic elements it contains, and especially with 

 water in the greatest quantity and constancy. The latter property ensures 

 a luxuriant vegetation ; whilst the former, on account of its favouring the 

 chemical processes in the plant, ensures an opulence of form which is 

 quite impossible in a poorer soil. Indeed, we never see in the virgin 

 soils of nature, nor in our fields, the rich variety of forms which are 

 observed in our gardens, and in some instances the action of the cir- 

 cumstances producing these varieties is so permanent that they can be 

 propagated by seeds. It is impossible that these influences should lose 

 their activity even where they are formed without the assistance of man. 

 Thus, we find in the tropics, where the conditions fail for forming a good 

 garden soil, that there, as with us, wildernesses, or a wearisome mono- 

 tonous vegetation, prevail. On the other hand, we find that where in 

 the tropics the conditions of a rich garden soil prevail, that there we 

 have the greatest profusion of forms and the most luxuriant vegetation. 

 In this way many varieties produced in the course of centuries may be- 

 come permanent forms, whilst the forms of a less favoured climate may 

 be only the residue of an earlier period in the history of the earth : so 

 also in higher latitudes, even at the poles, the peculiarities of the atmo- 

 sphere may produce conditions which are now only found under the 

 tropics. 



