KNOWLEDGE AND OPINION. 



are known, catalogued, fmislied, and done for. 

 That the earth revolves round the sun in a cer- 

 tain known number of days, hours, minutes, and 

 seconds ; that the moon revolves in an equally 

 known period round the earth ; that Jupiter and 

 Saturn have respectively so many satellites or 

 attendant moons; that Neptune exercises a cer- 

 tain measurable influence on the orbits of the 

 other planets, — these are facts now definitely set- 

 tled once and for ever. But what is the exact 

 constitution of the sun's body, what is the precise 

 nature of comet's tails, how the meteors are re- 

 lated to the planetary system, and so forth, — 

 these are questions still engaging the attention of 

 astronomers, and on which different autliorities 

 are as yet liable to express different opinions. 

 Similarly with other sciences. That bats, in spite 

 of their wings, display immense similarity of 

 structure to hedgehogs and shrewmice ; that bam- 

 boos are only very large and woody grasses ; that 

 the brain in man is the organ of mind ; that the 

 use of the heart is to set the blood in circulation, 

 — these are facts universally admitted. But what 

 is the wild plant from which we derive our cultiva- 

 ted wheat, whether all animals are descended from 

 a common ancestor, what is the particular func- 

 tion of each part of the brain, how the blood acts 

 in building up the various nerves and muscles, — 

 these are questions on which men of science have 



