NEGATIVE EVIDENCE. 83 



was sown in a hot-bed. The young plants 

 were set out, some in very rich soil, some in 

 stiff, poor clay, some in old peat, and others in 

 pots in the greenhouse, seven hundred and 

 sixty-five in all. Though they and their parents 

 were subjected to all this diversity of treat- 

 ment, "not one of them presented the least 

 variation except in size." Negative evidence 

 is indeed of little value, unless it can be shown 

 that it covers the whole ground. In order to 

 transform these experiments into proof it would 

 be necessary to show that, if the three forms 

 belong to one species, the cowslip should have 

 varied under the conditions to which it was 

 subjected. It is far more difficult to disprove 

 a proposition by negative evidence than by 

 proving the truth of its contradictory. Darwin 

 accordingly demonstrated what several other 

 botanists had surmised: that the oxlip is a 

 hybrid between the cowslip and the primrose. 



His efforts to determine whether Orchis morio 

 secretes nectar also furnish a good illustration 

 of his treatment of negative evidence. 1 A 

 nectary implies nectar, but Sprengel had thor- 

 oughly searched many flowers of O. maculata 

 and morio, and could not find a drop. Of 

 his own efforts in this direction, Darwin said, 



1 Fertilization of Orchids, pp. 36-41. 



