Kṛṣṇa says in the Bhagavad-gītā that sarva-yoniṣu kaunteya sambhavanti mūrtayo yāḥ (BG 14.4). In every forms of life, as many living entities are there... I have explained, there are different forms of life. Even within the water there are 900,000 forms of life. This is Vedic knowledge. You can take directly knowledge. You can understand. You do not require to dive into the water and make study, aquatic research work. You take the knowledge from the Vedic literature. You immediately understand that there are 900,000 species of life. This is different forms. The living entity, soul, is everywhere. But according to his karma... Just like nowadays people are very fond of diving within the water and swim. This has become a fashion. So next life they are going to become fish. Yes. Because yaṁ yaṁ vāpi smaran bhāvaṁ tyajaty ante kalevaram (BG 8.6). If you at the time of death, if you think of that, how to swim very nicely within the water, that means next life nature will give you a fish life. You get it. That is God's mercy. Why you artificially try to become a fish? You become actually fish. That is nature's gift. So you'll get. Yaṁ yaṁ vāpi smaran. This is stated in the Bhagavad... Because whatever we practice in our life, so that concept of life, that imagination, continues.
Just like you dream. What do you dream? Whatever you think always, you dream like that. It is mental activities. So those who are trying to become fishlike, nature offers him a actual body of fish. That is transmigration of the soul. We are desiring different types of desires and we are getting a body. This is nature's law. Just like you infect some type of contagious disease. You suffer from that disease. Nature's way. It will be automatically manufactured. That is the disease. Similarly, the body will be automatically manufactured. This is the nature's law. Prakṛteḥ kriyamāṇāni guṇaiḥ karmāṇi sarvaśaḥ (BG 3.27). You are under the control of the material nature and if you are manufacturing different ideas, nature will give you the facility to have such body. That is called transmigration of the soul. Therefore we find so many varieties of forms of life within the water, on the land, so many plants, so many varieties of trees, so many varieties of insect, so many varieties of birds, beast, and then human life, then civilized life.