But both the Amphibia and the Fishes reveal a lower plane of the sexual instincts, when the sexes, dominated by some imperious instinct, gather in hordes, commingling to shed their precious germs into the surrounding water, there to effect the work of fertilization and the achievement of new birth. The all-important union of these germs is no mere work of chance, as it might seem, but the sperms seek the ova with unerring surety, guided, in this case, by that very efficient substitute for instinct, chemotaxis, or the attraction which certain chemical substances have for lowly organized living bodies. In this case the allurement is furnished by the ova. It is surely no unreasonable surmise that here we have the beginnings of the complex phenomena which the earlier chapters have revealed. On this lower plane we are probably confronted by instinct alone, but from this level upwards intelligence plays an increasingly important part. CHAPTER X SOME OF THE “LOWER ORDERS” Butterflies and Moths, and the Coloration of their Wings—Female Choice and “Fine Feathers”—When Male Butterflies are Dominant—Sexual Selection among Butterflies—Abortive Experiments—Wallace and the Sexual Selection Theory—The Sense of Smell in Butterflies and Moths—Fragrant Butterflies—Wingless Moths and their Lures to Lovers—Methods of Pairing among Butterflies and Moths—More Experiments. Not the least impressive feature met with in the study of animal behaviour under the spell of the Sexual Instincts is its uniformity. This fact becomes the more apparent as one turns to the lower grades of life. Whether one starts with the vertebrates and works downwards, or vice versa, the same problems arise and the same interpretation is demanded. That is to say, the theory of “Sexual Selection” leads one to the same conclusions whether it be tested by the evidence afforded by the Butterflies and Moths, or that furnished by Birds or Mammals. The accessory phenomena, the vehicles which give expression to these internal fires, are in like manner curiously similar. These “vehicles” are the “secondary sexual characters”—colour, and armature, and scent. These very tangible signs are the phenomena in the Mystery Play of Sex which first catch the attention of the investigator. To account for these the theory of “Sexual Selection” was first devised. After the birds, probably the group most conspicuous for its splendour is that which contains the Scale-winged Insects or Lepidoptera, and it has always been allowed that any explanation of the one must apply also to the other. It seems impossible to avoid this conclusion. But before going further it would be well to take note of one or two interesting features in regard to coloration that have so far not been touched upon in these pages.