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Show 188 INSTINCT. [Cu.A.P. VII. "Natura non facit saltum " applies with almost equal force to instincts as to bodily organs. Changes of instinct ~a;y sometimes be facilitated by the same species having different instincts at different periods of life, or at different seasons of the year, or when placed under different circuinstances, &c. ; in which case either one or the other instinct might be preserved by natural selection. And such instances of diversity of instinct in the san1e species can be shown to occur in natute. Again as in the case of corporeal structure and confonnably_ with my theory, the instinct of each 'species is good for Itself, but has never, as far as we can judge been produced for the exclusive good of others. One ~f the strong~st instances of an animal apparently performinO' an action for the sole good of another, with which I a: acquainted, is that of aphides voluntarily yielding their sweet excretion to ants : that they do so voluntarily the following facts show. I removed all the ants fro~n a group of about a dozen aphides on a dock-plant, and prevented their attendance during several hours. After this interval, I felt sure that the aphides would want to excrete. I watched them for some time through a lens, but not one excreted; I then tickled and stroked thmn with a l1air in the same manner, as well as I could, as the ants do with their antennre ; but not one excreted. Afterwards I allowed an ant to visit them, and it im1nediately seemed, by its eager ·way of running about, to be well aware what ~ rich flock it had discovered; it then began to play with 1ts antennre on the abdomen first of one aphis and then of another; and each aphis, as soon as it felt the antennre, immediately lifted up its abdomen and excreted a limpid drop of sweet juice, which was eagerly devoured by the ant. Even the quite young aphides behaved in this man- · ner, showing that the action was instinctive, and not the result of experience. But as the excretion is extren1ely viscid, it is probably a convenience to the aphides to have it removed.; and therefore probably the aphides do not instinctively excrete for the sole good of the ants. Although I do not believe that any animal in the world performs an action for the exclusive good of another of a 0HAP. VII.] INSTINCT. 189 distinct species, yet each species tries to take advantage of the instincts of others, as each takes advantage 'of the weaker bodily structure of others. So again, in some few cases, certain instincts cannot be considered as absolutely perfect ; but as details on this and other such points are not indispensable, they may be here passed over. As so1ne degree of variation in instincts under a state of nature, and the inheritance of such variations, are indispensable for the action of natural selection, as many instances as possible ought to have been here given; but want of space prevents me. I can only assert, that instincts certainly do vary-for instance, the migratory instinct, both in extent and direction, and in its total loss. So it is with the nests of birds, which vary partly in dependence on the situations chosen, and on the nature and temperature of the country inhabited, but often from causes wholly unknown to us : Audubon has given several remarkable cases of differences in nests of the same species in the northern and southern United States. Fear of any particular ene1ny is certainly an instinctive quality, as may be seen in nestling birds, though it is strengthened by experience, and by the sight of fear of the same enemy in other animals. But fear of man is slowly acquired, as I have elsewhere shown, by various animals inhabiting desert islands; and we may see an instance of this, even in England, in the greater wildness of all our large birds than of our small birds; for the large birds have been most persecuted by man. We may safely attribute the greater wildness of our large birds to this cause ; for in uninhabited islands large birds are not more fearful than small ; and the magpie, so wary in England, is tame in Norway, as is the hooded crow in Egypt. That the general disposition of individuals of the same species, born in a state of nature, is extremely diversified, can be shown by a multitude of facts. Severa~ cases also, could be given, of occasional and strange habits in certain species, which might, if advantageous to the species, give rise, through natural selection, to quite new in- . a tincts. But I am well aware that these general statements, without facts given in detail, can produce but a feeble |