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[insertion by Henrietta Litchfield:] end of Chapter VIII of Origin p. 297

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Sect 7. Instincts

"natura non facit saltum"" has is applicable to instincts as well as to corporeal structure, & is plainly explicable on our theory views, but is otherwise, as it seems to me, inexplicable,— all tend to corroborate the theory of inheritance & natural selection. So again does the very general fact of closely allied, but certainly distinct species, when inhabiting distant parts of the world, & exposed to considerably different conditions of life, yet often retaining nearly the same instincts. For instance we can understand on the principle of inheritance, how it is that the Thrush of South America lines its nest with mud in the same peculiar manner as does our British thrush; how it is that the male Troglodyte of North America, builds "cock-nests", like the males of our Kitty-wrens, "cock-nests" to roost in, — a habit unlike that of any other Known birds. Finally, 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 larvæ of ichneumonidæ feeding within the live live bodies of caterpillars,— not as instincts specially given by the Creator, but as very small parts consequences of of one general law, leading to the advancement of all organic beings;— namely, multiply, vary, let the strongest Live & the weakest Die.)

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[mathematical notes by George Darwin, lower half of page in another hand, not transcribed]