It may not be quite easy for you to anticipate why I regard endogamy as a key to the mystery of the Caste system. Not to strain your imagination too much, I will proceed to give you my reasons for it.
It may not also be out of place to emphasize at this moment that no civilized society of to-day presents more survivals of primitive times than does the Indian society. Its religion is essentially primitive and its tribal code, in spite of the advance of time and civilization, operates in all its pristine vigour even to-day. One of these primitive survivals, to which I wish particularly to draw your attention, is the *custom of exogamy*. The prevalence of exogamy in the primitive world is a fact too well known to need any explanation. With the growth of history, however, exogamy has lost its efficacy and, excepting the nearest blood-kins, there is usually no social bar restricting the field of marriage. But regarding the peoples of India the law of exogamy is a positive injunction even to-day. Indian society still savours of the clan system, even though there are no clans: and this can be easily seen from the law of matrimony which centres round the principle of exogamy, for it is not that _sapindas_ (blood-kins) cannot marry, but a marriage even between _sagotras_ (of the same class) is regarded as a sacrilege.
Nothing is therefore more important for you to remember than the fact that endogamy is foreign to the people of India. The various _gotras_ of India are and have been exogamous: so are the other groups with totemic organization. It is no exaggeration to say that with the people of India exogamy is a creed and none dare infringe it, so much so that, in spite of the endogamy of the Castes within them, exogamy is strictly observed and that there are more rigorous penalties for violating exogamy than there are for violating endogamy. You will, therefore, readily see that with exogamy as the rule there could be no Castes, for exogamy means fusion. But we _have_ Castes; consequently in the final analysis creation of Castes, so far as India is concerned, means the superposition of endogamy on exogamy. However, in an originally exogamous population an easy working out of endogamy (which is equivalent to the creation of Caste) is a grave problem, and it is in the consideration of the means utilized for the preservation of endogamy against exogamy that we may hope to find the solution of our problem.