What it amounts to is that no particular factors seem to be able to explain this directional ability of theirs. Not the response to changing currents in the spring sea, not the perception by fish of varying pressures in salt water, or of differences in salinity, nor their possible ability to use the sun as a reference point in navigation ... none of these approaches have yet solved the great mystery. Do they have some special sense, some perceptiveness, about which we know nothing? Scientists have measured and probed their reactions for a long time, but so far have not found any evidence of a special sensory ability. Biologically, fish do have several unique characteristics. For example, they have an “air bladder” by means of which they are able to adjust themselves to changing densities in the water. They also possess a “lateral line” organ, consisting of a tube or canal under the skin filled with mucus and connected to the nervous system. This sense, closely associated with hearing, enables them to detect vibrations of a very low intensity in the water and to avoid obstacles, such as an approaching bank or another fish. Aside from that, fish can smell, they have sight, and they have a sense of touch and taste. These known senses are what scientists count on in investigating the migratory behavior of fish. They test their responses to different stimuli. On that basis, one of the most recent directions to be explored centers around the environmental factors which the fish are subjected to, such as currents, temperatures, the physical and chemical nature of the waters through which they swim. These factors are supposed to guide them successively on their migrations and to be so consistent year after year that the responsive fish return to their streams of origin because they never got off the track. Different schools, or age groups, of alewives would go to separate streams, because they responded differently, as Gerald B. Collins puts it in his study of alewives at Bournedale, “to the existing patterns of environmental stimuli.” Homing, from the environmentalists’ point of view, is neither a matter of memory nor mystery. I do not have enough knowledge behind me to discuss such a method or approach, but it does seem to have the advantage of comprehensiveness, of taking the whole journey in. It does not depend on any single factor to explain migratory behavior, and it provides a good long track of exploration, step by step. Whether the migrant fish behave as mechanically as this suggests, or whether the factors involved are separately either as consistent as they are supposed to be, or amount in the aggregate to as much as they should, remains to be seen. We are still in the realm of theory, however rationally expressed, and do not know yet how the fish find their destination.