(Abbreviation SB = Srimad Bhagavatam)
Analogy of the body
According to the Vedas (Rg Veda 10.90 etc.) the four varnas are created from the body of the Lord's universal form (virat-purusa). Thus their roles are analogous to the bodily limbs whose proper cooperation is essential for the healthy body.
"The virat-purusa's face is the brahmanas, His arms are the ksatriyas, His thighs are the vaisyas, and the sudras are under the protection of His feet. All the worshipable demigods are also overtaken by Him, and it is the duty of everyone to perform sacrifices with feasible goods to appease the Lord." (SB 2.1.37)
"The brahmanas represent His mouth, the ksatriyas His arms, the vaisyas His thighs, and the sudras are born of His legs." (SB 2.5.37)
"The brahmanas and Vedic knowledge come from the mouth of the Supreme Personality of Godhead, the ksatriyas and bodily strength come from His arms, the vaisyas and their expert knowledge in productivity and wealth come from His thighs, and the sudras, who are outside of Vedic knowledge, come from His feet. May that Supreme Personality of Godhead who is full in prowess, be pleased with us." (SB 8.5.41)
Ship analogy: brahmana is a helmsman, ksatriya a captain, vaisya the cooks and sudra the sailors. Only if they cooperate, the ship can reach its destination.
Jai Sri Krishna.