Answer :
Because of the isolation of Galapagos, the life forms that had somehow reached the island, such as the lizard and other such creatures, had evolved differently over time compared to their cousins on the mainland. Things like a bird that was usually able to fly no longer had long wings spans or the ability to fly on the island, the large lizards on the island were used to eating sea-vegetation and often swam right into the ocean using their long tails to swim. And so forth.
Darwin found that all the plant life and animals on the island developed from isolation. Each of these species were also different form the other species on the islands surrounding the Galapagos islands. Darwin could only explain these characteristics through a gradual transformation of the various species.
