In his book The Singularity Is Near, Ray Kurzweil eloquently summarizes some extremely interesting phenomena about the evolution of technology and how it fits with grand scale evolution. I have read this book twice and am fascinated with the concepts contained in it and their prospective application to the financial markets.
He suggests that an exponential trend will cross "the knee of the curve" before it becomes recognizable to the average observer. If you consider the fact that the underlying trend in the Dow Jones Industrial Average is exponential in nature, I would place the knee of the curve somewhere during the first third of the 20th century. In other words, we're well past it and the trend will continue much higher in an accelerating fashion.
