"Instinct" is the seventh chapter of On the Origin of Species by Means of Natural Selection by Charles Darwin. Darwin starts this chapter by explaining why it is not part of the previous chapter, "Difficulties on Theory," or woven into other previous chapters. "The subject of instinct," he writes, "might have been worked into the previous chapters; but I have thought that it would be more convenient to treat the subject separately, especially as so wonderful an instinct as that of the hive-bee making its cells will probably have occurred to many readers, as a difficulty sufficient to overthrow my whole theory." He also notes that, as with life generally, he has "nothing to do with the origin of primary mental powers," but is concerned only with the evolution of "diversities of instinct and of other mental qualities of animals."
The main purpose of this chapter is to suggest that, like its physical structure, the instincts of a species evolve through natural selection. Darwin has a few special points to make about this. One is that "[n]o complex instinct can possibly be produced through natural selection, except by the slow and gradual accumulation of numerous, slight, yet profitable, variations." The second is that "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."
With this, Darwin jumps into the topic of insect instincts, starting with those of aphids and ants. He notes that "[o]ne of the strongest instances of an animal apparently performing an action for the sole good of another… is that of aphides voluntarily yielding their sweet excretion to ants." But, as Steve Jones explains in his book Darwin's Ghost, the relationship between ants and aphids is not a one-sided one; "the aphids of those species, lacking soldiers of their own, trust in the jaws of the ants to keep their enemies at bay." Through this relationship, both aphids and ants gain something.
Other ants, however, have slave-making instincts. These ants enslave ants of other species to do their work for them. One of these species is so reliant on its slaves that the males and fertile females do no work, while the sterile females only capture more slaves. All digging, food-gathering, feeding, and raising of young is done by the slave species. Darwin notes that Pierre Huber did experiments on this species. He "shut up thirty of them without a slave, but with plenty of the food which they like best, and with their larvae and pupae to stimulate them to work," and they did nothing. They did not even eat, "and many perished of hunger." Other slave-making ants are not so reliant on their slaves, but keep them nonetheless.
Darwin adds that an evolution from ants that do not make slaves, all the way to the extremes of ants that die out without them, is not hard to imagine. He notes that ants that do not normally make slaves will sometimes carry off pupae of other species for food. If these pupae hatched, they would begin working out of their own innate instinct. "If their presence proved useful to the species which had seized them—if it were more advantageous to this species to capture workers than to procreate them—the habit of collecting pupae originally for food might by natural selection be strengthened and rendered permanent for the very different purpose of raising slaves." In other words, if it is easier for one species of ant to capture slaves of other species than to raise their own young, they may continue capturing other species, and this might affect the evolution of the master species.
This chapter also talks about a different kind of insect. It includes a long section called "Cell making instinct of the Hive-Bee." In Jones? version of this section, he writes about the bee?s ability to build orderly structures through the use of simple rules. "Bee society has no plans," he writes. "All that is needed is for each bee to know what goes on in its neighborhood, to pass its knowledge to others, and to have a threshold at which it changes from one behavior to another." For instance, bee hives have three regions; a central brood area, a rim of pollen cells surrounding it, and an outside wall of cells filled with honey. This orderly structure is the result of bees following their simple instincts. The queen bee, Jones writes, "follows a simple rule—lay an egg close to a cell that is already full…. This soon leads to a mass of eggs near the center, where she will more often recross her path." The workers also follow rules. "They store pollen and honey at random," Jones continues, "but remove it more quickly from cells near a larva. Pollen is used at ten times the rate of honey and in places with more brood cells it runs out faster. This leads to more turnover in a pollen cell, while those that contain honey, the long-term reserve, are soon blocked. The single place left for a pollen delivery is near the brood, where demand soon empties each cavity for reuse. To make the three-level pattern of the comb all that is needed is for each bee to test the contents of the cell next door."
In Darwin?s section on bees, he notes that, according to mathematicians who have studied beehives, hive bees "have made their cells of the proper shape to hold the greatest possible amount of honey, with the least possible consumption of precious wax in their construction." Darwin explains his observations of various sorts of bees, the most important of which is that various sorts of bees have hives of varying degrees of excellence, which he suggests shows that they are at various stages of evolution. He then asks a question: "how [could] a long and graduated succession of modified architectural instincts… have profited the progenitors of the hive bee?" The "answer is not difficult," he writes. Bees consume from twelve to fifteen pounds of sugar to create a single pound of wax. In addition, it takes a great deal of time to make the wax. Hence, wax has great value to bees and anything that might save them some increases their chances of survival and their ability to multiply. The advanced cell-making instincts of modern hive bees "can be explained by natural selection having taken advantage of numerous, successive, slight modifications of simpler instincts."
Darwin also looks at a possible origin for the European cuckoo?s habit of laying her eggs in other birds? nests. The American cuckoo does not have this habit, and instead hatches her own eggs. Darwin suggests that "the ancient progenitor of our European cuckoo had the habits of the American cuckoo; but that occasionally she laid an egg in another bird?s nest." If the young laid in the nests of other birds had a better chance of surviving than those laid in the mother cuckoo?s own nest, the habit of laying eggs in other?s nests might, through natural selection, become more common among cuckoos. The one question left is why birds raised by mothers of other species would have better chances of survival than those raised by their own mothers. The key to this is the reason for the European cuckoo?s habit: like the American cuckoo, "she lays her eggs, not daily, but at intervals of two or three days; so that, if she were to make her own nest and sit on her own eggs, those first laid would have to be left for some time unincubated, or there would be eggs and young birds of different ages in the same nest." Hence, the mother cuckoo has a very difficult time raising young in her own nest, and a good number of them do not survive, while those raised in the nest of a species that lays its eggs all at once are better cared for. By chance, the American cuckoo never developed this habit.
Darwin ends this chapter by writing that "it may not be a logical deduction, but to my imagination it is far more satisfactory to look at such instincts as the young cuckoo ejecting its foster-brothers,—ants making slaves,—the larvae of ichneumonidae feeding within the live bodies of caterpillars,—not as specially endowed or created instincts, but as small consequences of one general law, leading to the advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die."
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