On the Origin of Species1 is, in Charles Darwin’s words, “one long argument” about his perspective that all the species on Earth have evolved from “descent with modification through natural selection”. Reading it, I have the clear picture of an inquisitive, hard working, and intelligent man, who some have called one of the smartest men to have ever lived, having come to his hypothesis about speciation at a time when very little was understood about the biology of reproduction or the geology associated with the hundreds of millions of years during which life evolved on Earth from archaea to mammalian species. He argues this hypothesis at a time when as he says, “It is so easy to hide our ignorance under such expressions as ‘plan of creation,’ ‘unity of design,’ etc., and think that we give an explanation when we only restate a fact.” As most who have completed basic study in biology know, and only fundamentalists would argue against, his hypothesis has become a proven theory of evolution through natural selection, making this writing by Charles Darwin a demonstration of his exceptional powers of observation and analysis.
Much of this reading goes beyond my technical knowledge, as he refers to plant and animal orders, classes, families, genera and species to which I have no familiarity. Furthermore, he goes into exceptional depth on plant and animal morphology, geological record, biogeography, embryology, systematics, etc. demonstrating an exceptional breadth of knowledge of biology. With this mastery of these fields as background he explains thousands of details to support his hypothesis about natural selection which he has seen himself through simple observation or experimentation, or from reading publications or having direct communications with other naturalists, geologists, biologists, etc. So I am not attempting to provide even an outline of the work, but leaving it to the reader to scan this work, which Amazon provides free for Kindle software or as an Audible book.
In this his first and primary treatise on natural selection, he avoids stating that he thinks humans, themselves have evolved from other species, which he pursues, I’m sure in as clear and convincing a fashion, in The Descent of Man2 that he published a few years after he proposed the role of natural selection in On the Origin of Species.1 This gave the scientific community a little time to think through his arguments, and, relatively quickly, come to general agreement, with many resistant naturalists and others still insisting that a Creator fashioned all species, without giving adequate argument for some of the difficulties associated with this hypothesis, in comparison with the detailed arguments provided by Darwin.
Darwin closes with a clear statement of his view:
There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless form most beautiful and most wonderful have been, and are being, evolved.
Charles DARWIN. “On the origin of the Species,” pg 433.
References
- Darwin, Charles (1859). On the Origin of Species. London: John Murray.
- Darwin, Charles (1871). The Descent of Man, and Selection in Relation to Sex. John Murray.
T.P.Caruso 
The Darwin-Wallace Law of Evolution has been reworked and modified so extensively, that reading Darwin’s original works (Voyage of the Beagle, and The Origin of Species….) should only be done to provide an introduction or a historical perspective. Read current literature or even a good introductory text for a course on evolution to see how the Darwin-Wallace work has itself evolved.
Incidentally, notice I use the more accurate name – the Darwin-Wallace Theory. It seems everyone outside of systematists (like myself) and evolutionary biologists tends to forget poor Alfred Russell Wallace who independently came to the same conclusions as Darwin. Charles Lyell and Joseph Hooker arranged for Darwin and Wallace to present their theories together to a meeting of the Linnaean Society in 1858, the year before Darwin published his book. Also, the full title of the book as published by Darwin in 1859 is “On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.”