"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."

A seminal work and greatly influential on me personally, both in fostering my love of science and in guiding my career as a scientist. I reread Origin every few years and find wonder anew each time. I strongly recommend the first edition, despite it's flaws; it contains the purest form of Darwin's theories, which were somewhat adulterated in response to criticisms for the later versions, especially the sixth. Harvard University Press has a wonderful facsimile of the first edition, with an introduction by Ernst Mayr, that is well worth the extra couple of dollars you'll spend on it.