Experiment on fish is difficult, nevertheless it has been definitely proved that they do smell, and it seems probable that the sense is used by them for food-perception. Moreover, that it may be highly sensitive seems likely from the fact that sharks (which belong to the same order as dog-fish) can be attracted from great distances to putrid meat thrown into the water as bait, the high dilution of which resembles the behaviour of odour in an air medium. The belief that life in water, however, is less favourable than life on land to the fullest development of the sense is supported by the fact we have already mentioned that mammals living in water are extremely microsmatic. In the macrosmatic terrestrial animals not only is the olfactory sense relatively highly organised, but it is absolutely the predominant sense. Vision is subsidiary to it. In their brains the olfactory region constitutes by far the largest component. (The same, by the way, is true of the Reptilia.) In other words, it is upon the olfactory sense that these animals chiefly depend for their knowledge of the world. By it they are directed to their food, warned of their enemies, and attracted to their mates. Their universe is a universe of odour. In order to become more intimate with the details of this part of our subject, we shall pass in review some of the olfactory habits and characteristics of the macrosmatic animal most familiar to us, namely, the dog. There can be no doubt of the all-important part that smell plays in the life of the dog. Every one is familiar with it, and yet we do not often stop to think what its meaning is for the canine brain and understanding. One of the mysteries that must, one would suppose, for ever remain hidden from us, is what aspect the world we both share in company bears to this our closest animal friend. Who can tell what is passing through his mind as he sniffs at us? He can recognise his master by sight, no doubt, yet, as we know, he is never perfectly satisfied until he has taken stock also of the scent, the more precisely to do so bringing his snout into actual contact with the person he is examining. It is as if his eyes might deceive him, but never his nose. The greyhound courses by sight, but all other dogs hunt by scent, and the speed and certainty of foxhounds in full cry bear a new significance when we recollect that it is scent that is directing them. Could vision be any more swift and sure? We may heartily wish, as a child once remarked to a friend of mine, that Rover had a prettier way of saying “How d’ye do?” to his canine friends. But that and other even more objectionable habits do not prevent his _entrée_ into the most exclusive circles of human society. He is taken at his own valuation, and that, to be sure, is considerable. But the minute, the meticulous, olfactory scrutiny he makes of other dogs is but one more example of the predominance of this sense in his brain. (See also later.)