16 THE UNFATHOMED UNIVERSE 



Yet, as a warning against finality in another direction, we 

 must admit the vagueness that is apt to invest our knowledge 

 of the past. In the perfectly exact sciences, we can some- 

 times work backwards with remarkable certainty to comets, 

 eclipses, and the like; but in Biology how watchful we have 

 to be lest we get entangled in the vicious circle of inventing 

 a past from its continued life in the present, and then in- 

 terpreting the present in terms of our invention. 



(c) A reasonable humility of mind is also engendered 

 by recognising how limited, after all, is our range of exact 

 data. The late Professor Rowland, a distinguished physi- 

 cist, writes (1899, p. 408) : " In time we are limited by a 

 few hundred or possibly thousand years. ... In space we 

 have exact knowledge limited to portions of our earth's sur- 

 face and a mile or so below the surface, together with what 

 little we can learn from looking through powerful telescopes 

 into the space beyond. In temperature our knowledge ex- 

 tends from near the absolute zero to that of the sun, but 

 exact knowledge is far more limited. In pressures we go 

 from the Crookes vacuum, still containing myriads of flying 

 atoms, to pressures limited by the strength of steel, but still 

 very minute compared with the pressures at the centre of 

 the earth and sun, where the hardest steel would flow like 

 the most limpid water. In velocities we are limited to a few 

 miles per second. In forces to possibly one hundred tons to 

 the square inch. In mechanical rotations to a few hun- 

 dred times a second." Perhaps some of these limits have 

 been extended since Professor Rowland gave the address 

 from which we have quoted, but that would not affect our 

 point, the importance of bearing in mind the limits of 

 exact knowledge. 



(d) Another limitation is involved in the very nature 



