SB 3.6.32

viśo ’vartanta tasyorvor
 loka-vṛttikarīr vibhoḥ
vaiśyas tad-udbhavo vārtāṁ
 nṛṇāṁ yaḥ samavartayat
Word for word: 
viśaḥ — means of living by production and distribution; avartanta — generated; tasya — His (the gigantic form’s); ūrvoḥ — from the thighs; loka-vṛttikarīḥ — means of livelihood; vibhoḥ — of the Lord; vaiśyaḥ — the mercantile community; tat — their; udbhavaḥ — orientation; vārtām — means of living; nṛṇām — of all men; yaḥ — one who; samavartayat — executed.
Translation: 
The means of livelihood of all persons, namely production of grains and their distribution to the prajās, was generated from the thighs of the Lord’s gigantic form. The mercantile men who take charge of such execution are called vaiśyas.
Purport: 

Human society’s means of living is clearly mentioned here as viśa, or agriculture and the business of distributing agricultural products, which involves transport, banking, etc. Industry is an artificial means of livelihood, and large-scale industry especially is the source of all the problems of society. In Bhagavad-gītā also the duties of the vaiśyas, who are engaged in viśa, are stated as cow protection, agriculture and business. We have already discussed that the human being can safely depend on the cow and agricultural land for his livelihood.

The exchange of produce by banking and transportation is a branch of this type of living. The vaiśyas are divided into many subsections: some of them are called kṣetrī, or landowners, some are called kṛṣaṇa, or land tillers, some of them are called tila-vaṇik, or grain raisers, some are called gandha-vaṇik, or merchants in spices, and some are called suvarṇa-vaṇik, or merchants in gold and banking. The brāhmaṇas are the teachers and spiritual masters, the kṣatriyas protect the citizens from the hands of thieves and miscreants, and the vaiśyas are in charge of production and distribution. The śūdras, the unintelligent class of men who cannot act independently in any of the above-mentioned activities, are meant for serving the three higher classes for their livelihood.

Formerly, the brāhmaṇas were given all the necessities of life by the kṣatriyas and vaiśyas because they had no time to spend making a living. The kṣatriyas would collect taxes from the vaiśyas and śūdras, but the brāhmaṇas were exempt from paying income tax or land revenue. That system of human society was so nice that there were no political, social and economic upheavals. The different castes, or varṇa classifications, are therefore essential for maintaining a peaceful human society.