The cycle of evolution of species, or the soul's journey through species. The different levels of spiritual elevation, from the lowest to the highest devotee.

Lord Krishna teaches us about the hierarchy of beings and the different species.

Among the creations produced by the two manifested energies [spirit and inert matter], those possessing life force [vegetables, grass, trees, and plants in general] dominate over inert matter, stone, earth, etc.

In turn, reptiles, worms, and snakes, which are capable of movement, surpass the immobile plants, and animals with developed intelligence are superior to even reptiles.

Human beings are superior to animals, and ghosts to human beings, because they do not have physical bodies.

Above the ghosts are the Gandharvas (celestial singers) and, even higher, the Siddhas (perfect beings), then the Kinnaras (superhuman beings with supernatural powers), and finally the asuras (unenlightened, evil beings, who can subdue the aforementioned beings).

Above the asuras come the celestial beings, ruled by Indra, the king of the heavens. The direct sons of Brahma, including King Daksa, prevail over Indra, and of Brahma's sons, Siva is the greatest. Since Siva is the son of Brahma, the latter is considered superior to him, but Brahma himself is subordinate to Me, the Supreme Lord.

However, since I Myself am favorable to learned sages, they are the greatest of all.

Animate beings are superior to inanimate objects, and among them, those in which signs of life are manifested are more evolved. As for animals endowed with developed consciousness, they are better than them, and above them are beings endowed with developed sense perception.

Among beings endowed with sense perception, those with developed taste are higher than those with developed only touch. But higher than them are those who can smell, and higher still are those who can hear.

Beings that can distinguish different shapes are higher than those who perceive sounds. Higher than them are those with toothy jaws, and higher still are those with many legs. But higher still are quadrupeds, and above all, humans.

Among humans, those whose social organization is based on individual qualities and activities prevail over others, and within such a society, intelligent men, referred to as “learned sages,” are the most evolved. But one must also distinguish as the best among the learned sages those who have studied the Vedas (the original holy scriptures, also called “the true gospel”), and among them, the one who knows the true purpose of the Vedas is the highest.