For equity investment, the sources of profit are capital gain, dividends and reinvestment income. 

Take for example the MSCI World Index from 31 May 1989 to 29 May 2009, capital gain represents 47.41% of the profit while dividends & reinvestment income consists of the remaining 52.59% of the profit.  

Dividends and reinvestment income are extremely important but it is often ignored and disregarded. For more information, read  HERE (login required, clients only)