What significance were the finches on the Galapagos Islands to Darwin's theory?

People refer to "Darwin's finches" from time to time as a symbol of evolution in the Galapagos Islands, but the father of evolutionary theory actually dropped the ball on those birds, collecting better details on mockingbirds, tortoises and other species.

Charles Darwin's observations, notes and collected organisms from the Galapagos Islands during his 5-year voyage on the Beagle resulted in his theory of evolution by natural selection, one of the best substantiated theories in the history of science.

He collected several finch species, including the warbler finch, sharp-beaked finch, ground finch, small tree finch, large tree finch, common cactus finch and large ground finch.