48 THE PRINCIPLES OF SCIENCE. 



less amount than these must exist, and we should state 

 our negative result with corresponding caution. We can 

 only disprove the existence of a quantitative phenomenon 

 by showing deductively, from the laws of nature, that if 

 present it would amount to a perceptible quantity. As 

 in the case of other negative arguments (vol. ii. p. 19) we 

 must demonstrate that the effect would appear, where it 

 is by experiment found not to appear. 



Limits of Experiment. 



It will be obvious that there are many operations of 

 nature which we are quite incapable of imitating in our 

 experiments. Our object is to study the conditions under 

 which a certain effect is produced ; but one of those con- 

 ditions may involve a great length of time. There are 

 instances on record of experiments extending over five or 

 ten years, and even over a large part of a lifetime ; but 

 such intervals of time are almost nothing to the tirm 

 during which nature may have been at work. ' The con- 

 tents of a mineral vein in Cornwall may have been under- 

 going gradual change for a million years or more. All 

 metamorphic rocks have doubtless endured high tempera- 

 ture and enormous pressure for almost inconceivable 

 periods of time, so that chemical geology is general b 

 beyond the scope of experiment. 



Arguments have been continually brought againsl 

 Darwin's theory, founded upon the absence of any cleai 

 instance of the production of a new species. During 

 an historical period of perhaps four thousand years, IK 

 animal, it is said, has been so much domesticated as t( 

 become different in species. It might as well be argued, 

 as it seems to me, that no geological changes are taking 

 place, because no new mountain has risen in Great Britain 

 within the memory of man. Our actual experience of 



