Today, the complete lineage of the Inner Tantra can only be found in Tibetan Buddhism. It prevailed in India up until some eight or nine hundred years ago; when the Tibetans went to India to learn Buddhadharma, it was already difficult to come upon traces of the lineage. In ancient India, practitioners of the Inner Tantra were very discreet, the reason being that India at the time was a very complex place with people holding myriad and opposing views everywhere, a condition ripe for intense argument to erupt easily. Even Sakyamuni Buddha checked his words carefully when propagating the Dharma.
~Depicted from GATEWAY TO VAJRAYANA PATH -The Parable of a Black Snake