xviii.l OBSERVATION. 413 



and Ccelenterata, may break down by the discovery of in- 

 termediate or anomalous forms. As civilisation spreads 

 over the surface of the earth, and unexplored tracts are 

 gradually diminished, negative conclusions will increase 

 in force ; but we have much to learn yet concerning the 

 depths of the ocean, almost wholly unexamined as they 

 are, and covering three-fourths of the earth's surface. 



In geology there are many statements to which con- 

 siderable probability attaches on account of the large 

 extent of the investigations already made, as, for instance, 

 that true coal is found only in rocks of a particular geolo- 

 gical epoch ; that gold occurs in secondary and tertiary 

 strata only in exceedingly small quantities, 1 probably 

 derived from the disintegration of eajlier rocks. In 

 natural history negative conclusions are exceedingly 

 treacherous and unsatisfactory. The utmost patience 

 will not enable a microscopist or the observer of any 

 living thing to watch the behaviour of the organism under 

 all circumstances continuously for a great length of time. 

 There is always a chance therefore that the critical act or 

 change may take place when the observer's eyes are with- 

 drawn. This certainly happens in some cases ; for though 

 the fertilisation 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 

 himself adopted one conclusion on negative evidence, 

 namely, that the Orchis pyramidalis and certain other 

 orchidaceous flowers secrete no nectar. But his caution 

 and unwearying patience in verifying the conclusion 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 takes place 

 rapidly and might happen at early dawn, that inconvenient 

 hour of observation was specially adopted. Flowers of 

 different ages were subjected to irritating vapours, to mois- 

 ture, and to every condition likely to bring on the secretion ; 

 and only after invariable failure of this exhaustive inquiry 

 was the barrenness of the nectaries assumed to be proved. 2 



1 Murchison's Siluria, ist ed. p. 432. 



* Darwin's Fertilisation of Orchids, p. 48. 



