单项选择题

Where the Galapagos Islands lie today, there was once an unbroken expanse of Pacific. Then the sea began to seethe and simmer, the earth"s crust was pierced, and matter gushed forth con-tinuously. Lava and scoriae (loose, cinderlike lava) piled up until one day spewing volcanoes raised peaks above the brawling sea.
Many thousands of years passed before life could settle upon the cooled land. Then wind and waves brought the first hardy seed of primitive plants. After these came others, then insects, liz-ards, and birds. The chance that any animal would survive the trip from the South American con-tinent more than six hundred miles away was slight; only a few hardy species came through alive. This is why there are many gaps in the animal life of the islands.
Those that did reach the out-of-the-way islands developed over the ages into a number of quite peculiar forms. Because conditions in the Galapagos differed from those in their original homes, the animals had to change or perish. Only in the Galapagos are there seaweed-eating ma-rine lizards, giant tortoises, and flightless cormorants. In fact, most of the species found in the Galapagos Islands are endemics, they exist only in the archipelago. There are gaps in the animal life today because ______.