CC Madhya 6.156
The material energy acts on the living entity in different degrees, according to how he acquires the association of the three modes of material nature. There are 8,400,000 species of life, some inferior, some superior and some mediocre. The gradations of the bodies are calculated according to the covering of material energy. In the lower categories — including aquatics, trees, plants, insects, birds and so forth — spiritual consciousness is almost nonexistent. In the mediocre category — the human form of life — spiritual consciousness is comparatively awakened. In the superior life forms, spiritual consciousness is fully awakened. Then the living entity understands his real position and tries to escape the influence of material energy by developing Kṛṣṇa consciousness.