[填空题]The annals of natural history contain many astonishing examples of the ability of animals to find their way home after making distant journeys. Salmon, for example, are born in freshwater streams and soon afterward journey down to sea. Several years later, after they have attained maturity, they swim back upstream to spawn and, in many cases, to die. The particular stream that serves as the journey’s end is almost invariably the same one in which they were born. It is chosen out of dozens or hundreds of equally suitable streams. The expression "almost invariably" is used advisedly in this case. In one investigation by Canadian biologists, 469,326 young sockeye salmon were marked in a tributary of the Fraser River. Several years later almost 11,000 were recovered after they had completed a return journey to the very same stream, but not a single one was ever recovered from other streams nearby. What underwater guideposts can these fish possibly follow It has been discovered by A. D. Ha