366 THE SELE-EERTILIZATION OF PLANTS. 



the Christian era* {sic!), to the present day; and it now 

 prevails in England, France, and Germany. A saga- 

 cious observer, Bradley, writing-in 1724, says, 'When 

 we once become Masters of a good Sort of Seed, we 

 should at least put it into Two or Three Hands, where 

 the Soils and Situations are as different as possible; 

 and every Year, the Parties should change with one 

 another, by which Means, I find the Goodness of the 

 Seed will be maintained for several Years. For Want 

 of this Use, many Farmers have failed in their Crops, 

 been great Losers.' He then gives his own practical 

 experience on this head. A modern writer asserts, 

 ' Nothing can be more clearly established in agricul- 

 ture, than that the continual growth of any one variety 

 in the same district, makes it liable to deteriorate either 

 in quality or quantity.' Another writer states that he 

 sowed close together, in the same field, two lots of 

 wheat-seed, the product of the same original stock, 

 one of which had been grown on the same land, and 

 the other at a distance, and the difference in favor of 

 the crop from the latter seed was remarkable. A gen- 

 tleman, in Surrey, who has long made it his business 

 to raise wheat to sell for seed, and who has constantly 

 realized in the market, higher prices than others, as- 

 sures me that he finds it indispensable continually to 

 change his seed ; and that for this purpose, he keeps 

 two farms differing much in soil and elevation." 



(Darwin thinks, that the good derived is due to the 

 change, per se /) 



If, on the other hand, plants are changed to a local- 

 ity, wanting in some of the conditions, present at the 

 former habitat, and which supplies none of the condi- 

 tions which may have been absent, or in meagre 



*The impression, with the writer, has ever been, that the " Chris- 

 tian era" lasted, at least, until the Origin of Species was published. 



