While on the H.M.S. Beagle, and later back home in London, Darwin had come across the ideas of the Rev. T.R. Malthus.
Malthus had realised that, although humans could double their population every 25 years, it did not happen in practice.
He thought the reason was that a struggle for existence (or resources) limited their numbers.
If numbers increased, then famine, wars and diseases caused more deaths.
Darwin, who knew that all living things could, in principle, increase their numbers, began to think about why some survived, while others did not.ᵖ²⁶⁴⁻²⁶⁸ His answer took years to develop.

The theory of evolution says that all living things on Earth, including plants, animals and microbes, come from a common ancestor by slowly changing throughout the generations.
Darwin suggested that the way living things changed over time is through natural selection.
This is the better survival and reproduction of those that best fit their environment.
Fitting into the place where you live is called adapting.
Those who fit best into the place where they live, the best adapted, have the best chance to survive and breed.
Those who are less well-adapted tend not to survive.
If they do not survive well enough to raise young, this means they do not pass on their genes.
In this way, the species gradually changes.

The first chapter of the Origin deals with domesticated animals, such as cattle and dogs.
Darwin reminded readers of the huge changes mankind had made in its domestic animals, which were once wild species.
The changes were brought about by selective breeding – choosing animals with desirable characters to breed from.
This had been done generation after generation, until our modern breeds were produced.
Perhaps what man had done deliberately, might happen in nature, where some would leave more offspring than others.

Darwin noticed that although young plants or animals are very similar to their parents, no two are exactly the same and there is always a range of shape, size, colour, and so on.
Some of these differences the plant or animal may have got from their own ancestors, but some are new and caused by mutations.
When such differences made an organism more able to live in the wild, it would have a better chance to survive, and would pass on its genes to its offspring, and they to their offspring.
Any difference that would cause the plant or animal to have less of a chance to live would be less likely to be passed on, and would eventually die out altogether.
In this way groups of similar plants or animals (called species) slowly change in shape and form so that they can live more successfully and have more offspring who will survive them.
So, natural selection had similarities to selective breeding, except that it would happen by itself, over a much longer time.

He first started thinking about this in 1838, but it took a full twenty years before his ideas became public.
By 1844 he was able to write a draft of the main ideas in his notebook.
Historians think that he did not talk about his theory because he was afraid of public criticism.
He knew his theory, which did not discuss religion, raised questions about the literal truth of the Book of Genesis.
Whatever the reason, he did not publish his theory in a book until 1859.
In 1858 he heard that another biologist, Alfred Russel Wallace, had the same ideas about natural selection.
Darwin and Wallace's ideas were first published in the Journal of the Linnaean Society in London, 1858.
Then, Darwin published his book the next year.
The name of the book was On the Origin of Species by means of Natural Selection, or the preservation of favoured races in the struggle for life.
This is usually called The Origin of Species.
While on the H.M.S. Beagle, and later back home in London, Darwin had come across the ideas of the Rev. T.R. Malthus. Malthus had realised that, although humans could double their population every 25 years, it did not happen in practice. He thought the reason was that a struggle for existence (or resources) limited their numbers. If numbers increased, then famine, wars and diseases caused more deaths. Darwin, who knew that all living things could, in principle, increase their numbers, began to think about why some survived, while others did not.ᵖ²⁶⁴⁻²⁶⁸ His answer took years to develop.
The theory of evolution says that all living things on Earth, including plants, animals and microbes, come from a common ancestor by slowly changing throughout the generations. Darwin suggested that the way living things changed over time is through natural selection. This is the better survival and reproduction of those that best fit their environment. Fitting into the place where you live is called adapting. Those who fit best into the place where they live, the best adapted, have the best chance to survive and breed. Those who are less well-adapted tend not to survive. If they do not survive well enough to raise young, this means they do not pass on their genes. In this way, the species gradually changes.
The first chapter of the Origin deals with domesticated animals, such as cattle and dogs. Darwin reminded readers of the huge changes mankind had made in its domestic animals, which were once wild species. The changes were brought about by selective breeding – choosing animals with desirable characters to breed from. This had been done generation after generation, until our modern breeds were produced. Perhaps what man had done deliberately, might happen in nature, where some would leave more offspring than others.
Darwin noticed that although young plants or animals are very similar to their parents, no two are exactly the same and there is always a range of shape, size, colour, and so on. Some of these differences the plant or animal may have got from their own ancestors, but some are new and caused by mutations. When such differences made an organism more able to live in the wild, it would have a better chance to survive, and would pass on its genes to its offspring, and they to their offspring. Any difference that would cause the plant or animal to have less of a chance to live would be less likely to be passed on, and would eventually die out altogether. In this way groups of similar plants or animals (called species) slowly change in shape and form so that they can live more successfully and have more offspring who will survive them. So, natural selection had similarities to selective breeding, except that it would happen by itself, over a much longer time.
He first started thinking about this in 1838, but it took a full twenty years before his ideas became public. By 1844 he was able to write a draft of the main ideas in his notebook. Historians think that he did not talk about his theory because he was afraid of public criticism. He knew his theory, which did not discuss religion, raised questions about the literal truth of the Book of Genesis. Whatever the reason, he did not publish his theory in a book until 1859. In 1858 he heard that another biologist, Alfred Russel Wallace, had the same ideas about natural selection. Darwin and Wallace's ideas were first published in the Journal of the Linnaean Society in London, 1858. Then, Darwin published his book the next year. The name of the book was On the Origin of Species by means of Natural Selection, or the preservation of favoured races in the struggle for life. This is usually called The Origin of Species.
Charles Darwin