14 THE PRINCIPLES OP AGRICULTURE 



not strange, therefore, that this science should be more inti- 

 mately associated than others with agricultural ideas ; but we 

 now understand that agriculture cannot be an exact or definite 

 science, and that the retort and the crucible can solve only a few 

 of its many problems. In particular, we must outgrow the idea 

 that by analyzing soil and plant we can determine what the one 

 will produce and what the other needs. Agricultural chemistry 

 is the product of laboratory methods. The results of thess 

 methods may not apply in the field, because the conditions there 

 are so different and so variable. The soil is the laboratory in 

 which the chemical activities take place, but conditions of 

 weather are ever modifying these activities ; and it is not always 

 that the soil and the plant are in condition to work together. 



20a. As an illustration of the agricultural interest which 

 attaches to the surface geology of a region, see Tarr's "Geo- 

 logical History of the Chautauqua Grape Belt," Bull. 109 Cor- 

 nell Exp. Sta. 



21a. Probably no less than 50,000 species of plants (or 

 forms which have been considered to be species) have been 

 cultivated. The greater number of these are ornamental sub- 

 jects. Of orchids alone, as many as 1,500 species have been 

 introduced into cultivation. Nicholson's Illustrated Dictionary of 

 Gardening describes about 40,000 species of domesticated plants. 

 Of plants grown for food, fiber, etc., De Candolle admits 247 spe- 

 cies (in Origin of Cultivated Plants), but these are only the most 

 prominent ones. Vilmorin (The Vegetable Garden) describes 

 211 species of kitchen -garden vegetables alone. Sturtevant 

 estimates (Agricultural Science, iii., 178) 1,076 species as having 

 been "recorded as cultivated for food use." Of some species, 

 the cultivated varieties are numbered by the thousands, as in 

 apple, chrysanthemum, carnation, potfito. Of animals, more 

 than 50 species are domesticated, and the breeds or varieties 

 of many of them (as in cattle) run into the hundreds. 



216. It is commonly said that agriculture is itself a science, 

 but we now see that this is not true. It has no field of science 

 exclusively its own. Its purpose is the making of a living for 

 its practitioner, not the extension of knowledge. The subject of 



