The change into another species is by the filling in of Nature

Verse 4.2

 जात्यन्तयऩणयिाभ् प्रकृत्याऩयातू ॥ २ ् ॥

jatyantaraparinamah prakrityapoorat

The change into another species is by the filling in of Nature.

Patanjali explained that nature's energy flows in such abundance in certain adepts that their consciousness is transformed, enabling them to live in a pure dynamic state in this present life. (If a sadhaka fails to reach perfection during this life, nature may abundantly penetrate and perfect his sadhana in the next, so that he may then experience freedom.)

Due to the power of his practices, nature's energy flows in such a sadhaka with such force as to transform him into an immortal. 

It is interesting to note also that nature itself is the power-house for spiritual evolution.

Sri Ramanujacharya , jnaneshvar, Tukaram and Nandanar attained kaivalya during their lives here on earth without yogic sadhana because of the fruits of yogic practices in their previous lives (jatyantara parnaima) .

Similarly, Acharyani's Maitreyi, Gargi, Arundhati, Lilavati, Sulabha, Sarada Devi, yogini Lalla of Kashmir and yogini Motibai of Jaipur, all attained

liberation through jatyantara parinama ( fruits of yogic practice in their previous lives)

PATANJALI YOGA SUTRAS