He says that he and others working the field expected to find a pretty big amount of genetic variation.

FORBES: All Mapped Out

There is also a certain amount of natural variation.

ECONOMIST: Cancer genetics

So for Darwin to think that natural selection explains major evolutionary innovations, he had to bet on the amount and kind of variation that typically exists in lineages.

ECONOMIST: A master of evolution takes a last long look at Darwin

But there was also great variation in the amount of rain.

BBC: Heatwave sees warmest UK April for more than 100 years