对此，有一种可能的解释是，洲际迁移既需要连接两个大陆的大陆桥，也需要大陆桥上和桥对面存在合适的栖息地。
A likely explanation for this involves the fact that intercontinental migrations require both a land bridge connecting the two continents and the suitable habitat both on and across that land bridge.
19世纪，国会为连接东西海岸的洲际铁路拨款。
In the 19th century Congress provided funding for a transcontinental railway linking the east and west coasts.
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