The power of influence, by sympathetic action, may also be seen in another direction; consider the fact that if we are in a room with a piano and we sing a certain note, say E flat, we not only hear that note coming back from the piano, but, if we examine the strings, we find that all the E flats are actually vibrating in sympathy, because they are in perfect harmony with the note given out by the voice; but none of the other strings are responding because they are out of harmony. With this simile in mind, let us consider the curious fact that a moth always lays its eggs on that particular plant upon which the caterpillars, when they hatch out of these eggs, must feed. The study of the Life History of Insects has always been of great interest to me, as I firmly believe that we are on the verge of a great discovery, and that the first indications are being revealed to us through the investigation of the Biology of Insects. Some of you may, perhaps, have watched this progress of ovipositing, as I have done, and noticed how the female moth will hover in a peculiar way over different plants, but does not alight until she comes to a plant near akin to the one she is seeking. She then alights, but remains, on tip-toe as it were, with legs outstretched and wings quivering, and soon mounts again into the air; it is only when she alights on the proper food plant that she shows unmistakably that she knows her quest is ended and her eggs are laid. This particular plant has no other attractions for her, she takes her food irrespectively from any other flower which secretes honey, and yet, when she is ready to fulfil her destiny, she is unerringly drawn towards that particular plant which must be the food of her offspring. What is this wonderful sense? We call it instinct, a name which is made to cover all other senses in the lower animals, of which we have no cognisance ourselves. Let us take our own senses as a guide: we find that they are all based on the appreciation of frequencies, of greater or less rapidity, by means of organs specially adapted to vibrate in sympathy with those pulsations, and thus we gain knowledge of external things. Two tuning forks or two organ pipes when vibrating close to each other, give out a pure musical note when they are in perfect harmony, and they then have, as it were, "rest" together; but when one is put even slightly out of harmony, there is, in place of a pure musical note, a rise and fall of sound in heavy throbs, strangely characteristic of "quarrelling"; in fact, discord and "unrest."