This paper was first published in the Journal in 1969 (Vol. 22, p. 118). It is followed by comments from John Kemp.

The last twenty-one years have seen some very striking advances in our knowledge of how animals can determine their location. In many cases we have learned that they have available a wider range of stimuli than ourselves for recognizing landmarks and for pilotage within their home area. Thus the associated senses of smell and taste are extraordinarily well developed in some species. The ability of the males of certain moths to detect the scent emitted by females at very considerable distances had long been known. More recently the extreme sensitivity, and selectivity, of fish to waterborne odours has led to an understanding of how they locate their home waters. As but one example, eels have shown reactions to concentrations of chemicals as low as 3 × 10−18, equivalent to but two or three molecules within the fish's olfactory sac. In other cases animals have developed sensitivities of which we have little or no conception. Ecolocation is employed by certain birds, by many marine animals and reaches a peak of efficiency in the case of bats. Not only do the latter detect sounds of much higher frequency than ourselves, they also respond to echoes of sounds they emitted but 0·001 seconds earlier. We have little appreciation of the sensations produced by the pressure-receptors in the lateral-line organs of fish.