Srimad Bhagavatam Canto 05, Chapter 19, Text 19

SB 5.19.19

asminn eva varse purusair labdha-janmabhih sukla-lohita-krsna-varnena svarabdhena karmana divya-manusa-naraka-gatayo bahvya atmana anupurvyena sarva hy eva sarvesam vidhiyante yatha-varna-vidhanam apavargas capi bhavati.
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
The people who take birth in this tract of land are divided according to the qualities of material nature — the modes of goodness [sattva-guna], passion [rajo-guna], and ignorance [tamo-guna]. Some of them are born as exalted personalities, some are ordinary human beings, and some are extremely abominable, for in Bharata-varsa one takes birth exactly according to one’s past karma. If one’s position is ascertained by a bona fide spiritual master and one is properly trained to engage in the service of Lord Visnu according to the four social divisions [brahmana, ksatriya, vaisya and sudra] and the four spiritual divisions [brahmacari, grhastha, vanaprastha and sannyasa], one’s life becomes perfect.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
For further information, one should refer to Bhagavad-gita (14.18 and 18.42-45). Srila Ramanujacarya writes in his book Vedarta-sangraha:
evam-vidha-parabhakti-svarupa-jñana-visesasyotpadakah purvoktaharahar upaciyamana-jñana-purvaka-karmanugrhita-bhakti-yoga eva; yathoktam bhagavata parasarena — varnasrameti. nikhila-jagad-uddharanayavanitale ’vatirnam para-brahma-bhutah purusottamah svayam etad uktavan — “svakarma-niratah siddhim yatha vindati tac chrnu” “yatah pravrttir bhutanam yena sarvam idam tatam/ svakarmana tam abhyarcya siddhim vindati manavah.”
In the Visnu Purana (3.8.9), the great sage Parasara Muni has recommended:
 purusena parah puman
visnur aradhyate pantha
 nanyat tat-tosa-karanam
“The Supreme Personality of Godhead, Lord Visnu, is worshiped by the proper execution of prescribed duties in the system of varna and asrama. There is no other way to satisfy the Lord.” In the land of Bharata-varsa, the institution of varnasrama-dharma may be easily adopted. At the present moment, certain demoniac sections of the population of Bharata-varsa are disregarding the system of varnasrama-dharma. Because there is no institution to teach people how to become brahmanas, ksatriyas, vaisyas and sudras or brahmacaris, grhasthas, vanaprasthas and sannyasis, these demons want a classless society. This is resulting in chaotic conditions. In the name of secular government, unqualified people are taking the supreme governmental posts. No one is being trained to act according to the principles of varnasrama-dharma, and thus people are becoming increasingly degraded and are heading in the direction of animal life. The real aim of life is liberation, but unfortunately the opportunity for liberation is being denied to people in general, and therefore their human lives are being spoiled. The Krsna consciousness movement, however, is being propagated all over the world to reestablish the varnasrama-dharma system and thus save human society from gliding down to hellish life.
Srimad Bhagavatam Canto 05, Chapter 19, Text 17-18
Srimad Bhagavatam Canto 05, Chapter 19, Text 20