18 THE PRINCIPLES OF SCIENCE. 



centuries 1 , and the probability that if existent they would 

 have been seen, increases the probability that they do not 

 exist. We may with somewhat less confidence discredit the 

 existence of any large unrecognised fish, or sea animals, 

 such as the alleged sea-serpent. But as we descend to 

 forms of smaller size negative evidence loses weight from 

 the less probability of our seeing smaller objects. Even 

 the strong induction in favour of the four-fold division of 

 the animal kingdom into Vertebrata, Armulosa, Mollusca 

 and Coelenterata, 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 in- 

 crease in force ; but we require to learn much yet con- 

 cerning the depths of the ocean, almost wholly unexaminec 

 as they are, and covering three-fourths of the earth's 

 surface. 



In geology there are a number of assertions to which 

 considerable probability attaches on account of the large 

 extent of the investigations already made, as, for in- 

 stance, that true coal is found only in rocks of a par- 

 ticular geological epoch ; that gold occurs in secondary 

 and tertiary strata only in exceedingly small quantities" 

 probably derived from the disintegration of earlier 

 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 any 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 



t Cuvier's 'Essay on the Theory of the Earth,' translation, p. 61, &c. 

 11 Murchisou's 'Siluria/ ist ed. p. 432. 



