In Bhagavad-gītā (BG 14.14-15) it is said that those who are highly developed in the mode of goodness are promoted to the higher, heavenly planetary system, and those who are overpowered by the mode of passion are situated in the middle planetary systems—the earth and similar planets. But those who are surcharged with the mode of ignorance are degraded to the lower planetary systems or to the animal kingdom. The demigods are highly developed in the mode of goodness, and thus they are situated in the heavenly planets. Below human beings are the animals, although some of them mingle with human society; cows, horses, dogs, etc., are habituated to living under the protection of human beings.
The word ātyantikena is very significant in this verse. By development of the mode of goodness of material nature one can become situated in the heavenly planets. But by excessive development of the modes of passion and ignorance, the human being indulges in killing the animals who are meant to be protected by mankind. Persons who indulge in unnecessary animal killing have excessively developed in the modes of passion and ignorance and have no hope of advancing to the mode of goodness; they are destined to be degraded to lower statuses of life. The planetary systems are calculated as upper and lower in terms of the classes of living entities who live there.