Something pricked or punctured into one. Distinguish is of the same root, and means to prick or puncture separately. Extinguish means to prick or puncture out. IL all cases the allusion is to marking by a puncture. At college the “markers” at the chapel doors still hold a pin in one hand, and prick with it the name of each “man” who enters. The word is used to express a natural impulse to do something; an inherent habit.
“Although reason may ... be blended with instinct the distinction
between the two is sufficiently precise. Reason only acts upon a
defluite and often laboriously acquired knowledge of the relation
between means and ends.” —