PREFACE. xix 



process of adaptation of plants to climates and soils, by nat- 

 ural selection and the survival of the fittest. The natural 

 floras and sylvas are thus the expression of secular, or rather, 

 millennial experience, which if rightly interpreted must convey 

 to the cultivator of the soil the same information that other- 

 wise he must acquire by long and costly personal experience. 



The general correctness of this axiom is almost self-evi- 

 dent ; it is explicitly recognized in the universal practice of 

 settlers in new regions, of selecting lands in accordance with 

 the character of the forest growth thereon; it is even legally 

 recognized by the valuation of lands upon the same basis, for 

 purposes of assessment, as is practiced in a number of States. 



The accuracy with which experienced farmers judge of the 

 quality of timbered lands by their forest growth, has justly 

 excited the wonder and envy of agricultural investigators, 

 whose researches, based upon incomplete theoretical assump- 

 tions, failed to convey to them any such practical insight. It 

 was doubtless this state of the case that led a distinguished 

 writer on agriculture to remark, nearly half a century ago, 

 that he " would rather trust an old farmer for his judgment 

 of land than the best chemist alive." * 



It is certainly true that mere physico-chemical analyses, un- 

 assisted by other data, will frequently lead to a wholly errone- 

 ous estimate of a soil's agricultural value, when applied to cul- 

 tivated lands. But the matter assumes a very different as- 

 pect when, with the natural vegetation and the corresponding 

 cultural experience as guides, we seek for the factors upon 

 which the observed natural selection of plants depends, by 

 the physical and chemical examination of the respective soils. 

 It is further obvious that, these factors being once known, 

 we shall be justified in applying them to those cases in which 

 the guiding mark of native vegetation is absent, as the result 

 of causes that have not materially altered the natural condition 

 of the soil. 



It is probable that, had agricultural science been first de- 

 veloped in regions where the external conditions permitted 

 the carrying-out of such a course of investigation, instead of 

 in the abnormally temperate, even and humid climate of middle 



1 " The Soil Analyses of the Geological Surveys of Kentucky and Arkansas." 

 S. W. Johnson in AM, JOUR. SCI., Sept. 1861. 



