As one progressively moves down the scale from satya yuga to kali yuga, the power of righteous diminishes and evil starts to triumph. In treta yuga people were righteous. The system of varnashrama dharma goes back to treta yuga. This is typified in the principle of four varnas (classes) and four ashramas (stages of life). The four varnas are brhamanas, kshatriyas, vaishyas and shudras. Each class has its proper place, each its assigned occupations as determined by the principles of division of labour. The four ashramas are brhamacharya (celibate studenthood). Garhasthya (householder stage), vanaprastha (forest-dwelling stage), and sannyasa (hermithood). Everyone followed the tenets of varnashrama aharma in treta yuga. Individuals were equally handsome and equally wealthy. Poverty and disease were unknown. The weather was clement and, initialy, there was no need to build houses. Cities and villages were not known. People lived freely in the mountains and on the shores of the oceans. But as people became evil, the weather turned inclement. Houses had to built so the people might be protected from the heat and the cold. Cities and villages were planned and constructed. Evil became even more prevalent in dvapara yuga. In some cases, individuals began to practise that which was against the dictates of varnashrama dharma. To bring men back to the righteous path, the knowledge of the sacred Vedas had to be disseminated amongst humans. To this end, the great Vedavyasa divided the Vedas into four. Various other shastras (sacred texts) were also composed in dvapara yuga. These include ayurveda (medicine), jyotisha (astronomy) and arthashastra (economics). Hatred, jealousy, warfare and other evils first originated in dvapara yuga. Individuals did not stick to their own classes while marrying, and cross-breeds started to be born. But some remnants of righteousness could still be found, so that average life expectancies amounted to two thousand years. Kali yuga is the worst of the four eras. Theft, hatred, falsehood, fraudulence and egotism become the norm. As a natural corollary to such evils, drought and famine recur again and again. Event the brahmans are led astray. They do not study the Vedas, nor do they perform yajnas. The brahmanas degenerate so much that they mix with shudras. As for the shudras, they become kings. Can anything more ridiculous be imagined? Shudras even start to study the Vedas. Evil always has its effect. Disease becomes rampant and life expectancies go down.