OBSERVATION. 13 



probably have perished entirely, distilled perhaps into 

 oils. It has been pointed out that the peculiar character 

 of some extinct floras may be due to the unequal preser- 

 vation of different families of plants. By various acci- 

 dents, however, we may gain glimpses of a world that 

 is usually lost to us as by insects embedded in amber, 

 the great mammoth preserved in ice, mummies, casts in 

 solid material like that of the Roman soldier at Pompeii, 

 and so forth. 



We should also remember, that just as there may be 

 conjunctions of the heavenly bodies that can have hap- 

 pened only once or twice in the period of history, so re- 

 markable terrestrial conjunctions may take place. Great 

 storms, earthquakes, volcanic eruptions, landslips, floods, 

 irruptions of the sea may, or rather must, have occurred, 

 events of such unusual magnitude and such extreme rarity 

 that we can neither expect to witness them nor readily 

 to comprehend their effects. It is a great advantage of 

 the study of probabilities, as Laplace himself remarked, to 

 make us mistrust the extent of our knowledge, and pay 

 proper regard to the probability that events would come 

 within the sphere of our observations. 



Apparent' Sequence of Events. 



De Morgan has excellently pointed out 1 that there 

 are no less than four modes in which one event may 

 seem to follow or be connected with another, without 

 being really so. These involve mental, sensual, and ex- 

 ternal causes of error, and I will briefly state and illustrate 

 them. 



Instead of A causing B, it may be our perception of A 

 that causes B. Thus it is that prophecies, presentiments, 



l ' Essay on Probabilities/ Cabinet Cyclopaedia, p. 121. 



