Darwin's early notebooks discussed how non-adaptive characteristics could be selected when animals or humans chose mates, [196] with races of humans differing over ideas of beauty. [197] In his 1856 notes responding to Robert Knox's The Races of Man: A Fragment, he called this effect sexual selection. [198] He added notes on sexual selection to his "big book on species", and in mid-1857 he added a section heading "Theory applied to Races of Man", but did not add text on this topic. [199] It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working'

All the foregoing rules and aids and difficulties in classification are explained, if I do not greatly deceive myself, on the view that the natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking,... [168] In Chapter II, Darwin specifies that the distinction between species and varieties is arbitrary, with experts disagreeing and changing their decisions when new forms were found. He concludes that "a well-marked variety may be justly called an incipient species" and that "species are only strongly marked and permanent varieties". [121] He argues for the ubiquity of variation in nature. [122] Historians have noted that naturalists had long been aware that the individuals of a species differed from one another, but had generally considered such variations to be limited and unimportant deviations from the archetype of each species, that archetype being a fixed ideal in the mind of God. Darwin and Wallace made variation among individuals of the same species central to understanding the natural world. [117] Struggle for existence, natural selection, and divergence [ edit ] Yet the book also shows that the ultimate prevalence of the theory of evolution over rival forms of explanation did not come easily. Darwin had to think carefully how to convince his contemporaries of its validity. He had to defend himself against accusations of blasphemy; some of the resulting ridicule targeted him personally. There were serious scientific objections to the process of natural selection as the key mechanism of evolution, including Carl Nägeli's insistence that a trivial characteristic with no adaptive advantage could not be developed by selection. Darwin conceded that these could be linked to adaptive characteristics. His estimate that the age of the Earth allowed gradual evolution was disputed by William Thomson (later awarded the title Lord Kelvin), who calculated that it had cooled in less than 100 million years. Darwin accepted blending inheritance, but Fleeming Jenkin calculated that as it mixed traits, natural selection could not accumulate useful traits. Darwin tried to meet these objections in the fifth edition. Mivart supported directed evolution, and compiled scientific and religious objections to natural selection. In response, Darwin made considerable changes to the sixth edition. The problems of the age of the Earth and heredity were only resolved in the 20th century. [89] [236]Throughout his writing, Darwin sought to counter potential adverse responses to his theory with an onslaught of fact. On the Origin of Species is peppered with examples from the natural world illustrating the principles of evolutionary theory in practice. More detail was given in Darwin's 1868 book on The Variation of Animals and Plants Under Domestication, which tried to explain heredity through his hypothesis of pangenesis. Although Darwin had privately questioned blending inheritance, he struggled with the theoretical difficulty that novel individual variations would tend to blend into a population. However, inherited variation could be seen, [138] and Darwin's concept of selection working on a population with a range of small variations was workable. [139] It was not until the modern evolutionary synthesis in the 1930s and 1940s that a model of heredity became completely integrated with a model of variation. [140] This modern evolutionary synthesis had been dubbed Neo Darwinian Evolution because it encompasses Charles Darwin's theories of evolution with Gregor Mendel's theories of genetic inheritance. [141] Difficulties for the theory [ edit ]

A particular nasty interpretation of Darwin’s theory came to be known as "social Darwinism”. It transferred the ideas of a “struggle for existence” and the “survival of the fittest” to human society, where they were used as an argument against social benefits for the poor and disadvantaged. In the most serious consequence, this lead to racism, eugenics, forced sterilisations, and the euthanasia of “unfit” people. During the 3 years he spent at Cambridge, he did become acquainted with the rudiments of botany and a bit of geology, but he judged the time mostly wasted. He occupied himself with beetle collecting and dinner parties—not unknown to Cambridge students today, except for the beetle collecting.

An ominous absence

I think all the grand leading facts of geographical distribution are explicable on the theory of migration (generally of the more dominant forms of life), together with subsequent modification and the multiplication of new forms. We can thus understand the high importance of barriers, whether of land or water, which separate our several zoological and botanical provinces. We can thus understand the localisation of sub-genera, genera, and families; and how it is that under different latitudes, for instance in South America, the inhabitants of the plains and mountains, of the forests, marshes, and deserts, are in so mysterious a manner linked together by affinity, and are likewise linked to the extinct beings which formerly inhabited the same continent... On these same principles, we can understand, as I have endeavoured to show, why oceanic islands should have few inhabitants, but of these a great number should be endemic or peculiar;... [167] Classification, morphology, embryology, rudimentary organs [ edit ] Chapter XIII starts by observing that classification depends on species being grouped together in a Taxonomy, a multilevel system of groups and sub-groups based on varying degrees of resemblance. After discussing classification issues, Darwin concludes: Within a day or two of formulating this teleological argument, Darwin opened up his N Notebook, in which he began constructing his theory of human moral evolution. As the above passages indicate, he considered moral behavior to be a species of social instinct. One difficulty he recognized immediately was that the social instincts benefited not their carriers but their recipients. This meant that his new device of natural selection would not appear to provide their account, which is probably why Darwin initially relied on the inheritance of acquired habit to explain these innate behaviors. Darwin would apply his device of natural selection to explicate moral behavior only after he had solved a significant problem that threatened to overturn his entire theory—or at least he so judged.

methodological canons (Lennox 2005). One prominent way Darwin captured the complexity of this process is reflected in the single diagram to appear in all the I believe I was considered by my [school] masters and by my Father as a very ordinary boy, rather below the common standard in intellect. To my deep mortification my father once said to me, “You care for nothing but shooting, dogs, and rat-catching, and you will be a disgrace to yourself and all your family.Darwinian evolution, under the aegis of natural selection, is also progressive. As Darwin expresses it in the penultimate paragraph of the book: “And as natural selection works solely by and for the good of each being, all corporal and mental endowments will tend to progress toward perfection” (Darwin, 1859, p. 489). This kind of progress is not merely local. In chapter 10 of the Origin, for instance, Darwin asserts that “the more recent forms [of creatures] must, on my theory, be higher than the more ancient; for each new species is formed by having had some advantage in the struggle for life over other and preceding forms” (1859, pp. 336–337). This is a universal proposition, not confined to a local population. He then provides an operational test—at least in imagination—of this consequence. If Eocene creatures adapted to a particular environment were put in competition with modern animals, Darwin conjectures, “the Eocene fauna or flora would certainly be beaten and exterminated” (1859, p. 337). He assumes that the accumulation of improvements would give the advantage to more progressive (i.e., recent) creatures—even if compared with animals adapted to the same environment. This presumption of cumulative adaptational advantage, of course, does not play a role in neo-Darwinian theory. But then, as I’ve pedantically argued, Darwin was not a neo-Darwinian. I might have adduced for this same purpose the differences between the races of man, which are so strongly marked; I may add that some little light can apparently be thrown on the origin of these differences, chiefly through sexual selection of a particular kind, but without here entering on copious details my reasoning would appear frivolous." [200] See also: Reactions to On the Origin of Species In the 1870s, British caricatures of Darwin with a non-human ape body contributed to the identification of evolutionism with Darwinism. [204]