Srimad Bhagavatam Canto 07, Chapter 11, Text 18-20


SB 7.11.18-20

rtamrtabhyam jiveta
 mrtena pramrtena va
satyanrtabhyam api va
 na sva-vrttya kadacana
rtam uñchasilam proktam
 amrtam yad ayacitam
mrtam tu nitya-yacña syat
 pramrtam karsanam smrtam
satyanrtam ca vanijyam
 sva-vrttir nica-sevanam
varjayet tam sada vipro
 rajanyas ca jugupsitam
sarva-vedamayo viprah
 sarva-devamayo nrpah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
In time of emergency, one may accept any of the various types of professions known as rta, amrta, mrta, pramrta and satyanrta, but one should not at any time accept the profession of a dog. The profession of uñchasila, collecting grains from the field, is called rta. Collecting without begging is called amrta, begging grains is called mrta, tilling the ground is called pramrta, and trade is called satyanrta. Engaging in the service of low-grade persons, however, is called sva-vrtti, the profession of the dogs. Specifically, brahmanas and ksatriyas should not engage in the low and abominable service of sudras. Brahmanas should be well acquainted with all the Vedic knowledge, and ksatriyas should be well acquainted with the worship of demigods.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
As stated in Bhagavad-gita (4.13), catur-varnyam maya srstam guna-karma-vibhagasah: the four divisions of human society were created by the Supreme Lord according to the three modes of material nature and the work ascribed to them. Formerly, the principle of dividing human society into four sections — brahmana, ksatriya, vaisya and sudra — was strictly followed, but because of gradual neglect of the varnasrama principles, varna-sankara population developed, and the entire institution has now been lost. In this Age of Kali, practically everyone is a sudra (kalau sudra-sambhavah), and finding anyone who is a brahmana, ksatriya or vaisya is very difficult. Although the Krsna consciousness movement is a movement of brahmanas and Vaisnavas, it is trying to reestablish the divine varnasrama institution, for without this division of society there cannot be peace and prosperity anywhere.
Srimad Bhagavatam Canto 07, Chapter 11, Text 17
Srimad Bhagavatam Canto 07, Chapter 11, Text 21