In the quest of phyletic lines, biologists who have based their opinions on the widest field of observation and comparison known to their age have often felt that the position which they have assigned to any organism within its own affinity may be given to another by later systematists.

The origin of this uncertainty of systematic position is not to be found in a foreboding that the immediate descendants of an organism, now considered relatively simple, may suddenly acquire characters which will justify changes of position in the affinity. It is to be found in the biologist's avowed ignorance of the relative value of the characters by which he judges, and in his belief that much which is important has escaped his observation.