There have been some areas in which the construction of dams across a stream or river has almost destroyed an old run, although there were a few fish left over, making a yearly, token migration. They continued to come in and spawn below the last, impassable obstruction in their way; but when the dams were abandoned and removed and new fishways built so that they could travel upstream to the headwaters, the alewives stayed where they were. They did not migrate beyond their original limit, and the population failed to increase. It was as if there were an invisible wall in the water where the last, accustomed barrier had stood. Not until the headwaters above were newly stocked with spawning fish was there any chance of the run being fully used. This built-in reaction to home waters might go back for thousands of years in an unobstructed stream, or, in a new run, only three or four. It has its limits, and its wisdom. What can go farther back, or forward, than its own birth? When I started watching the alewives I heard of one phenomenon that seemed to me to surpass all analysis. It tempted me to look for magic. The run that comes up from Nantucket Sound on the south side of the Cape by way of the Herring River goes through a pond called Hinckley’s, then through a stream ending in a fairly large body of water called Long Pond. The migrating alewives are also able to go into another pond out of Hinckley’s--north of it, and east of Long Pond--called Seymour Pond, but the majority seem to spawn in Long Pond, from which there is only one clear exit. The fish must go back to Hinckley’s, the way they came. But this outlet was not always the same. Early in the nineteenth century the natural outlet was a brook going into Seymour Pond, but it was blocked and banked off by the construction of some cranberry bogs, and the present outlet was dug some five hundred yards away. Now the extraordinary thing is that on their return, the alewives still school in the banked-up area of the old outlet. I went over there and could find nothing to distinguish it from the rest of the sandy, water-lapped shore, except that there was a slightly boggy area on the other side of the road where the brook used to run, and a trace of its route through the underbrush. Was there any reason at all why these fish should be able to detect an old route cut off a hundred years ago? The direction of its flow no longer existed as currents in the water. Yet the “damfool herrin’” were certainly behaving according to report. I could see them, a school of several hundred, running freely along the shore, slipping lightly over the sand through the unruffled pond waters. When they reached the area of the old outlet they began an almost puzzled circling, which continued for some time before they swung back again. There should have been a common-sense reason for it, but I was tempted to ask, “What is verifiable truth?” and not stay for an answer.