Cycle of Birth and Death
As the embodied soul continuously passes, in this body, from boyhood to youth to old age, the soul similarly passes into another body at death. A sober person is not bewildered by such a change.
The living entity in the material world carries his different conceptions of life from one body to another, as the air carries aromas. Thus he takes one kind of body and again quits it to take another.
Being engaged in all these materialistic activities, the living soul entangles himself in the cycle of the law of fruitive actions. This entails the chain of birth and death in the 8,400,000 species of life: the aquatics, the vegetables, the reptiles, the birds, the beasts, the uncivilized man, and then again the human form, which is the chance for getting out of the cycle of fruitive action.
Śrīmad-Bhāgavatam (2.1.5, Purport)