The Origin of Species: By Means of Natural Selection of the Preservation of Favoured Races in the Struggle for Life. –
-- Summary by Susan Fleck

Summary of Chapter 8 – Instincts:

1. Mental qualities of our domestic animals vary, and the variations are inherited

2. Instincts vary slightly in a state of nature

3. No one disputes that instincts are of the highest importance to each animal. Therefore, there is no real difficulty, under changing conditions of life, in natural selection accumulating to any extent slight modifications of instinct which are in any way useful.

4. In many cases habit or use and disuse have probably come into play. The facts given in this chapter do not strengthen in any great degree my theory; but none of the cases of difficulty annihilate it.

5. The facts that instincts are not always absolutely perfect and are liable to mistakes; that no instinct can be shown to have been produced for the good of other animals, though animals take advantage of the instincts of others—show that the canon in natural history of “Natura non facit saltum” is applicable to instincts as well as to corporeal structure—all tend to corroborate the theory of natural selection.

a. [“Natura non fact saltum” – Latin for Nature does not make jumps.” This has been a principle of natural philosophy since at least the time of Aristotle, around 400 BCE.]

6. It is common in the case of closely allied, but distinct species, that when inhabiting distant parts of the world and living under considerably different conditions of life, that they often retain nearly the same instincts.

a. Example: The thrush of tropical S. America lines its nest with mud in the same peculiar manner as does our British thrush

b. Example: The Hornbills of Africa and India have the same extraordinary instinct of plastering up and imprisoning the females in a hole in a tree, with only a small hole left in the plaster through which the males feed them and their young when hatched

7. 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.

a. Note: The Ichneumonidae are a family within the order Hymenoptera. Insects in this family are commonly called ichneumon wasps. Less exact terms are ichneumon flies. There are now known over 60,000 species occurring worldwide. Some species of ichneumon wasps lay their eggs in the ground, but most inject them directly into a host’s body, typically into a larva (such as a caterpillar) or pupa.