SB 11.25.21

upary upari gacchanti
 sattvena brāhmaṇā janāḥ
tamasādho ’dha ā-mukhyād
 rajasāntara-cāriṇaḥ
Word for word: 
upari upari — higher and higher; gacchanti — they go; sattvena — by the mode of goodness; brāhmaṇāḥ — persons dedicated to Vedic principles; janāḥ — such men; tamasā — by the mode of ignorance; adhaḥ adhaḥ — lower and lower; ā-mukhyāt — headfirst; rajasā — by the mode of passion; antara-cāriṇaḥ — remaining in intermediate situations.
Translation: 
Learned persons dedicated to Vedic culture are elevated by the mode of goodness to higher and higher positions. The mode of ignorance, on the other hand, forces one to fall headfirst into lower and lower births. And by the mode of passion one continues transmigrating through human bodies.
Purport: 

Śūdras, persons in the mode of ignorance, are generally in deep illusion about the purpose of life, accepting the gross material body as the self. Those in passion and ignorance are called vaiśyas and hanker intensely for wealth, whereas kṣatriyas, who are in the mode of passion, are eager for prestige and power. Those in the mode of goodness, however, hanker after perfect knowledge; they are therefore called brāhmaṇas. Such a person is promoted up to the supreme material position of Brahmaloka, the planet of Lord Brahmā. One who is in the mode of ignorance gradually falls to the level of unmoving species, such as trees and stones, while one in the mode of passion, filled with material desire but satisfying it within Vedic culture, is allowed to remain in human society.