Though it may be impossible to miss, this year commemorates the 200th anniversary of the birth of Charles Darwin, who has justly been called the father of modern biology for this development of the theory of the evolutionary development of species. At times contentious, the fact of evolution has been long established beyond any kind of reasonable doubt - all schools of scientific knowledge are convergent in this regard. But to understand the true importance of evolution for biology as a whole, one of my favorite essays is by Theodosius Dobzhansky: "Nothing in biology makes sense except in the light of evolution."
In any case, I happen to find myself reading a facsimile of the original edition of On the Origin of Species - which I have had for many years but never read end to end. Coincidentally, the New York Times has a posted a great selection of passages by Darwin: read and enjoy.