The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut [1896] at sacred-texts.com

20. And on account of observation.

It is, moreover, observed that out of the four classes of organic beings--viviparous animals, oviparous animals, animals springing from heat, and beings springing from germs (plants)--the two latter classes are produced without sexual intercourse, so that in their case no regard is had to the number of oblations. The same may therefore take place in other cases also.--But, an objection may here be raised, scripture speaks of those beings as belonging to three classes only, because there are three modes of origin only; 'That which springs from an egg, that which springs from a living being, that which springs from a germ' (Kh. Up. VI, 3, 1). How then can it be maintained that there are four classes?--To this objection the next Sutra replies.