Oscillator circuit with/without 1M resistor

Looking at the Arduino schematic I see they use a large 1M resistor in the simple oscillator circuit. On others from Sparkfun the resistor is omitted. Can someone comment on the reason for the resistor. Thanks

http://www.evpc.ca/oscillator.JPG

the resistor it there to bias the amplifier that makes up the oscillator, often it is not needed, but with an unlucky combination of xtal, temperature, etc. etc. the oscillator might not start without it