(Prabhupada's lecture - Śrīmad-Bhāgavatam 1.15.44 - Los Angeles, December 22, 1973)
Therefore real problem is that, that we are coming, we are accepting one type of body, either in the human society or animal society or tree society or dog society, accepting some body, karmaṇā daiva, according to our karma. There (are) so many varieties of life. Why so many varieties of...? What is the explanation? Just like there are so many varieties of apartment.
So what does this mean? You enter some apartment according to the payment you can provide. Therefore there are so many varieties of apartment. It is a commonsense affair. Otherwise all apartments would have been the same, of the same size, same quality, same... No. There are different tenants, they pay differently; therefore there are different kinds of apartment. Similarly, this is also apartment. Dehino 'smin yathā dehe [Bg. 2.13]. The spirit soul is within this apartment body.
So according to karma, according to payment, one has got American body, one has got African body, one has got Indian body, one has got this body, that body, dog's body, cat's body, tree's body. This is karma. This is karma. This is our real problem. And this human life is especially meant for solving this problem, not the problem of petrol. But they have forgotten this. They are so rascals, the real problem they have forgotten.