1 Gita Sloka Every Day - Chapter 18 - Moksha Sanyasa Yoga - Sloka 41

The entire content of this mail is from Shri V N Gopala Desikan's  Srimad Bhagavad Gita, published by Vishishtadvaita Research Centre, Chennai and The Bhagavad Gita by Swami Chidbhavananda published by Ramakrishna Mission

The ways and means of liberation are now expounded

The Four Fold Caste Sstem Explained 41-48

1 Gita Sloka  Every Day - Chapter 18 - Moksha Sanyasa Yoga - Sloka 41

Brahmana Kshatriya visham shudranam cha param tapa I

Karmani pravibhaktani svabhava prabhavaih gunaih II sloka 41

ब्राह्मण क्षत्रिय विशां शूद्राणां च परम् तप I 

कर्मणि प्रविभक्तानि स्वभाव प्रभावैः गुणैः ईईस्लोक 41

The duties of the brahmanas, kshatriyas and vaisyas as also the sudras, O scorcher of the foes, are distributed according to the gunas born of their own nature.

The previous karmas are the causes for people being born as brahmins and others. The sattva and other gunas present in a person also arise from his past karmas

In a brahmin, sattva guna is predominant, with less of rajas and tamas. In a kshatriya, rajoguna is predominant, with less of sattva and tamas. In a vaisya, tamoguna is predominant in a small degree. In a shudra tamoguna is predominant in a greater degree. The shastras thus mention the different qualities of different people.  

The duties pertaining to these castes are divided in accordance to the predominant gunas, arising from their nature, as explained above.The duties for the caste have thus been accordingly prescribed by the shastras.