On the Origin of Species by Charles Darwin

A cautious, reasoned and detailed argument for the theory of descent with modification due to the power of natural selection, which will be fascinating, dry or a mixture depending on a reader’s interests. Darwin marshals numerous lines of evidence, from flowers and bees, to dogs and ants, to birds and bats, to support his theory; which though partly informative, I already agreed with him and knew some of the content, thus the occasionally tedium. However, it was useful to experience such an examination of classification of species and varieties in which it was made abundantly clear that the terms were not precisely defined, agreed upon and used consistently. Darwin’s theory offered greater explanatory power than anything else put forth as well as falsifiability – the hallmarks of good science. Additionally, he used human selective breeding (artificial selection) as stepping stone for understanding how natural selection could work; a wise tactical move.
One should read Origin if they will be satisfied to learn just what it was that Darwin said that changed the world, what he knew, what he did not, and how he tried to overcome the gap between the two; he acknowledged the shortcomings and offered various hypotheses to resolve complications.
On the Origin of Species is truly an intelligent, insightful and sustained science-based argument that is as cogent as it was revolutionary.

Selected Quotations:
(Introduction)
“Although much remains obscure, and will long remain obscure, I can entertain no doubt, after the most deliberate study and dispassionate judgement of which I am capable, that the view which most naturalists entertain, and which I formerly entertained -- namely, that each species has been independently created -- is erroneous. I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. Furthermore, I am convinced that Natural Selection has been the main but not exclusive means of modification.”

(Chapter 1)
“The laws governing inheritance are quite unknown; no one can say why the same peculiarity in different individuals of the same species, and in individuals of different species, is sometimes inherited and sometimes not so; why the child often reverts in certain characters to its grandfather or grandmother or other much more remote ancestor; why a peculiarity is often transmitted from one sex to both sexes or to one sex alone, more commonly but not exclusively to the like sex.”

(Chapter 3)
"Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring."

"Throw up a handful of feathers, and all must fall to the ground according to definite laws; but how simple is this problem compared to the action and reaction of the innumerable plants and animals which have determined, in the course of centuries, the proportional numbers and kinds of trees now growing on the old Indian ruins!"

“… that the structure of every organic being is related, in the most essential yet often hidden manner, to that of all other organic beings, with which it comes into competition for food or residence, or from which it has to escape, or on which it preys.”

(One could see how this could be, and probably was, misused)
“All that we can do, is to keep steadily in mind that each organic being is striving to increase at a geometrical ratio; that each at some period of its life, during some season of the year, during each generation or at intervals, has to struggle for life, and to suffer great destruction. When we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply.”

(Chapter 14, ending)
"It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed 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 forms most beautiful and most wonderful have been, and are being, evolved."