Caste marks do not, in fact, exist. The caste system, of course, does but the concept has been grossly degraded by 19th century colonialist historians who saw only its surface rigidities and made sweeping generalizations, (condemnatory for the most part), based on too little knowledge and even less experience. It is however ironic, that they never saw the parallels with the European system of guilds that divided artisans into separate social and economic entities on the basis of their specialization and sub-specializations.
For that is in simplified terms what the caste system is all about - a stratified and hierarchical socio-economic organization of society that evolved as India's ancient civilizations, (with its own social order, moral and ritual codes), absorbed the nomadic, Sanskrit speaking Aryan populations who crossed the mountain passes from the steppes of Central Asia and settled in Northern India. The ancient Hindus, literally meaning the peoples of the valley of the Indus river, soon took on functions and specializations that had little to do with tilling the soil. The four castes developed out of necessity, for with the evolution of society it was no longer possible for the tiller of the soil to assume the functions of priest, warrior, merchant, and artisan all rolled in one.
A new way of life brought with it a need for governance and order, defence and conquest, learning and trade, labourers and artisans. Roles began to be defined and people were classified according to their function, occupation and economic place in society. Brahmins' were to be the spiritual and temporal guides, teachers and exponents of law; Kshatriya were the warriors, princes and kings - in short, the nobility; Vaishya, took on the tasks of agriculture and merchantry; and Shudra included individuals who performed service communities -- manual and agricultural labourers, artisans, masons, etc. No king was complete without his brahmin eminence grise and over the centuries the brahmins attained immense power, upholding the law as well as dispensing it. But power, they say, corrupts and today, although all hindu priests are brahmins, they no longer hold the people in thrall as they once did.
The 4th group, sudra, denotes the service communities - manual and agricultural labourers, artisans, masons, etc. Although they lived on the fringes of society, the "outcastes" or "untouchables", the 5th group in the hierarchy, were still very much a part of mainstream society as the tasks of scavenging, cleaning up after funerals, killing or hunting animals for food, working in leather and other unclean materials, all fell to them. Mahatma Gandhi in the 1940s renamed them harijan, which when literally translated means "the people of God". There was a 6th group too, the malecha, (outsiders, or foreigners) who, like the Greeks, Kushans, Scythians and other invaders who settled in India, were gradually absorbed in the varnas (caste system) according to their profession.
The word caste is not Indian but comes from the Portugese word casta (breed or race). The Sanskrit word applied to the groupings is varna, which means several things but is often interpreted to signify colour. In a verse from the first millennium epic, the Mahabharata, Brigu, the sage explains: "The brahmins are fair, the kshatriyas are reddish, the vaishyas yellow and the sudras are black."
According to available evidence, the majority of the people seems to be radically very mixed, and to quote the Mahabharata again, "If different colours indicate different castes, then all castes are mixed castes." The Hindus also believe that a man's varna is determined by his profession and deeds and not by his birth. Besides, the ancients were not racists. The truth of the matter probably lies in the fact that varna, like a lot of Sanskrit words, changes its meaning according to the context it is used in and can denote form, quality, class, category, race, merit or virtue.
Eventually, however, varna came to signify an endogamic group, its members linked by heredity, marriage, custom and profession. Professions became diversified with the evolution of society and whole groups of people took on a new identity which was associated with the economic activity of their gotra (clan) and became subdivisions of the varnas. The laws that govern the varnas, and particularly the taboo on inter-caste marriages, have maintained the "purity" of the "breed" thus denoting "caste."