SB 5.1.13

Every living entity who has come to this material world has come here for material enjoyment, but according to his own karma, activities, he must accept a certain type of body given to him by material nature under the order of the Supreme Personality of Godhead. As stated in Bhagavad-gītā (3.27) , prakṛteḥ kriyamāṇāni guṇaiḥ karmāṇi sarvaśaḥ: everything is being done by prakṛti, material nature, under the direction of the Supreme Lord. Modern scientists do not know why there are varieties of bodies in 8,400,000 forms. The fact is that all these bodies are ordained for the living entities by the Supreme Personality of Godhead according to the living entities’ desires. He gives the living entities freedom to act as they like, but on the other hand they must accept a body according to the reactions of their activities. Thus there are different types of bodies. Some living entities have short durations of life, whereas others live for fantastic durations. Every one of them, however, from Brahmā down to the ant, acts according to the direction of the Supreme Personality of Godhead, who is sitting in everyone’s heart. As confirmed in Bhagavad-gītā (15.15) :

sarvasya cāhaṁ hṛdi sanniviṣṭo mattaḥ smṛtir jñānam apohanaṁ ca

“I am seated in everyone’s heart, and from Me come remembrance. knowledge and forgetfulness.” It is not a fact, however, that the Supreme Personality of Godhead gives direction to certain living entities in one way and other living entities in another way. The truth is that every living entity has a certain desire, and the Supreme Lord gives him a chance to fulfill it. The best course, therefore, is to surrender unto the Supreme Personality of Godhead and act according to His desire. One who does so is liberated.