The authors! explanation of this difficult scientific and philosophical concept is smoothly written, and appears to be unusually thorough. But, there is too much tangential information presented, which- while it adds to the conversational flow- often ends in disorganized rambling, causing ornate obscurity to reign in place of plain clarity. Although two comparatively untouched topics- geological time and biological time- are covered, this study does not serve as a replacement for either Tannenbaum's Understanding Time (1958, p.608, J-194) or Zarchy's Wheel of Time (1957, p.776, J-286), two fine studies of this subject for young scientists.