On the other hand, odours exist in vast variety amongst plants and animals which have not acquired any special association or significance. We find that some organisms produce as a result of their chemical life material which oxidises and gives out light and so these organisms are "phosphorescent" without any consequence, good or bad, to themselves. And then we come upon others (as, for instance, the glow-worms and fire-flies) which have made use of this "accidental" quality, and produce phosphorescent light in special organs so as to attract the opposite sex. Again, we find that the red-coloured oxygen-seizing crystalline substance haemoglobin exists in the blood of a vast number of animals, and might as well be green or colourless for all the good its colour does them. Yet here and there the splendid red colour which this chemical gives to the blood becomes of great importance as a "decoration," or "sex-ornament." The comb of the domestic fowl, the wattles of the turkey, but above all the supreme beauty of the human race--the cherry-red lips and the crimson-blushing cheek of healthy youth--owe their wonderful colour to the red blood which flows through them. So at last the redness, of the oxygen-carrier is turned to account. So it must be also with odorous substances. Many have been called into existence, but few have been chosen in the long course of animal evolution and selected as the important means of repulsion or attraction. There are odorous substances attached to many of the lower animals which seem to have no significance, but just happen to be the result of necessary chemical changes, not aimed (so to speak) at their production. Of course, it is very difficult to form a certain and definite conclusion as to their uselessness as odours. For instance, nearly all the sponges when fresh and filled with living protoplasm have a curious smell which reminds one of that given off by a stick of phosphorus. Marine sponges have it and so has the beautiful green or flesh-coloured liver sponge (common on the wood of rafts and weirs in the Thames). A rather uncommon marine worm, called _Balanoglossus_ or the acorn worm, has a very strong and unpleasant smell like that of iodoform. In neither case is the nature of the odorous body known, nor its use to the animal suggested. Smelts smell like cucumbers: the green-bone fish and the mackerel smell alike. One of the common earth-worms has a strong aromatic smell, and the common snail, as well as the sea-hare and one of the cuttle-fishes (_Eledone_), smells like musk. Musk itself is produced, as a scent attracting the opposite sex, by several animals--musk-deer, musk-sheep, musk-rats. I am not now attempting to enumerate the well-recognised odours of animals such as are extracted from them by man in order to "opsonize" himself, but am pointing to the more obscure cases. There is not a very great or marked variety in the odours of fishes; but reptiles with their dry,