Darwin’s finches provide circumstantial evidence for the origin of a new species by means of geographical isolation . Consider three different forms of the large insectivorous tree-finch On the most southerly Galapagos island is a small dark form with a comparatively small beak. On another island to the northwest is a rather large and less barred form. On the central island is a yet larger and paler type with a larger. More parrot-like beak. Evidently these three forms had a common ancestor and evolved their differences in geographical isolation. The differences among them do not seem great enough to set them apart as separate species, and they would be classed as subspecies but for one curious circumstance: on the southernmost island the two extremes the small dark form and the largest pale from live side by side by side without merging