18:41
ब्राह्मणक्षत्रियविशां शूद्राणां च परन्तप ।
कर्माणि प्रविभक्तानि स्वभावप्रभवैर्गुणैः ॥१८- ४१॥
braahmaNakShatriyawishaaM shoodraaNaaM cha parantapa ।
karmaaNi prawibhaktaani swabhaawaprabhawairguNaiH ॥१८- ४१॥
Source: Automatic ITRANS Transliteration
Of brahmins, kshatriyas, vaishyas and shudras, O foe-scorcher,
The distinct activities [are] originated automatically by the gunas.
Source: Milind S. Pandit
The division of labor into the four cate-goies -- Braahmana, Kshatriya, Vaishya, and Shudra -- is also based on the Gunas inherent in peoples' nature (or the natural propensities, and not necessarily as one's birth right), O Arjuna. (See also 4.13)
Brahmanas, ksatriyas, vaisyas and sudras are distinguished by their qualities of work, O chastiser of the enemy, in accordance with the modes of nature.
Source: Bhagavad Gita As It Is