On the Origin of Species
Darwin's thesis, published in 1859, seems so clear now: OF COURSE species adapt to and change with their environment, and OBVIOUSLY the most useful characteristics are passed from one generation to the next. How else could it possibly be? Except that it wasn't always clear: established religion taught that God had created man and his fellow creatures, immutably, and He, rather than our environment, shaped us.
Darwin could only dare to question the perceived wisdom of his day by amassing evidence. On the Origin of Species, substantial though it is, was only an outline of a much larger work which he intended to publish, but never did. It was hugely controversial, and Darwin was ridiculed by many – yet his book changed the world. Scientific progress, such as DNA analysis, has refined and explained mechanisms which he could not have known, but his basic precepts remain. The Origin is essential reading.