OBSERVATION. 19 



the fertilization of orchids by agency of insects is proved 

 as well as any fact in natural history, Mr. Darwin has 

 never been able by the closest watching to detect an insect 

 in the performance of the operation. Mr. Darwin has him- 

 self, indeed, adopted one conclusion on purely negative 

 evidence, namely that the Orchis pyramidalis and certain 

 other orchidaceous flowers secrete no nectar. But his 

 caution and unwearying patience in verifying the con- 

 clusion give an impressive lesson to the observer. For 

 twenty-three consecutive days, as he tells us, he examined 

 flowers, in all states of the weather, at all hours, in various 

 localities. As the secretion in other flowers sometimes 

 rapidly takes place and might happen at Qarly dawn, that 

 inconvenient hour of observation was specially adopted. 

 Flowers of different ages were subjected to irritating 

 vapours, to water, and every condition likely to bring on 

 the secretion ; and only after the invariable failure of this 

 exhaustive inquiry was the barrenness of the nectaries 

 assumed to be proved x . 



In order that a negative argument founded on the non- 

 observation of an object shall have any considerable force, 

 it must be shown to be probable that the object if existent 

 would have been observed, and it is this probability which 

 defines the value of the negative conclusion. The failure 

 of astronomers to see the planet Vulcan, supposed by some 

 to exist within Mercury's orbit, is no sufficient disproof of 

 its existence. Similarly it would be very difficult, or even 

 impossible, to disprove the existence of a second satellite of 

 small size revolving round the earth. But if any person 

 make a particular assertion, assigning place and time, then 

 observation will either prove or disprove the alleged fact. 

 Thus if it is true that when a French observer professed 

 to have seen a planet on the sun's face, an observer in 

 Brazil was carefully scrutinizing the sun and failed to see 



x Darwin's ' Fertilization of Orchids,' p. 48. 

 C 2 



