Yoga Sutras of Patanjali translated by Yogi Kalinath - Chapter 4, Verse 2
Pada 4, Sutra 2
The transformation into another category of existence is due to the abundant flow of Nature.
Nature is the fundamental substratum, the world ground for existence to evolve. Rishi Patanjali is inferring here both evolution into another form which is more suitable for Self-realization (from animal (largely tied to its instincts) to human (a being endowed with Manas or mind leading to discernment)) as well as involution, the inner process of Self-revelation through Sadhana and proper understanding.
Yoga is the evolutionary science and alchemical art of transforming the human animal into a divine being using Nature as the Menstruum.