28 The Evolution Hypothesis. 



process liable to error. Even if we grant that the 

 mind is a quite perfect reasoning instrument, the 

 physicist is beset with liabilities to misconceptions 

 and mistakes. These sources of error may be grouped 

 tinder five heads, duration, extent, minuteness, com- 

 plexity, and imperfection of the organs of sense. 



(1.) Duration. The stretch of time which may be 

 called the period of observation is as nothing com- 

 pared with the supposed duration of the universe ; and 

 the period of skilled and reliable observation is still 

 less. All scientific conclusions are based on observa- 

 tions made at one point in a boundless reach of move- 

 ment and ceaselessly flowing change. Two sources of 

 doubt lie in every instance; (a) whether the whole 

 thing, or all that is essential to the right understand- 

 ing of it for the purpose in hand, has been seen on the 

 side within the field of observation ; and (b) whether 

 the facts observed adequately represent the whole 

 series, which is conceived as extending backward to a 

 measureless distance in time. Observation, to yield 

 & valid result, must conform fully to both conditions. 

 If there be room for questioning the exactness or 

 sufficiency of the facts observed, we are bound to refuse 

 acceptance to any doctrine built thereon, when it 

 transcends the immediate bounds of experience and 

 formulates a law for all time. The divergence from 

 reality may be infinitesimal at the outset, and within 

 the range of experience of no practical account, but 

 carried over the entire sweep of cosmic history, back- 



