"... it at once struck me that [...] favourable variations would tend to be preserved and unfavourable ones to be destroyed."

Last but not least, the familiar influences on Darwin are also worth considering. His father, Robert Waring Darwin was a physician, head of an accommodated country side English family. His mother died when he was only 8 years old. Young Charles Darwin was not a good student, he seemed too distracted by leisure activities such as specimen collection and later by fishing and hunting. By his father design Charles Darwin begun studying medicine, but he never finished. In his autobiography he recalls how he couldn't stand witnessing a couple of surgical procedures at that time ("long before the blessed days of chloroform" ..."The two cases fairly haunted me for many a long year"). 
On the other hand Darwin's grand father, Dr. Erasmus Darwin, zoologist and precursor of evolution theories played a positive influence on him. When he was 17 years old 
Darwin reads with great admiration his grand father's book "Zoonomia", finds out about Lamarck's ideas and gets interested in Geology. But not in medicine. Given the little success with this first career, Darwin's father, a good example of the species stepping twice on the same hole, decided to reorient the career of his son. Charles, obedient of his father's designs begun the studies that would have converted him into a clergyman priest. Fortunately, these theological studies also remained unfinished. It was during this time, though, when Charles Darwin begins attending classes of Natural History with interest and visiting the house of the theologist and botanist J.S. Henslow, whose influence ended up as we already know with Darwin in the Beagle.

Certainly not.  Nature's clues in the form of living species would still have been there, waiting as needed for a member of a self-aware species (ours, of course) to disentangle its 'secrets' and tell the rest of us. 
But how long could had waited human kind to do this in a Darwinless society?. Answering this question in a general way is impossible. Yet, we have access to a singular answer that corresponds to our particular chain of past events. We can find this answer in History books. Or even better in Darwin's biographies (History books are too busy with wars, battles, treaties and the extension of empires in space and time). 
So it happened that in 1858, when Darwin had already advanced in his analysis and interpretation of data and had already written the first ten chapters of his masterpiece, he received a letter of a young naturalist, also an Englishman, Mr. Alfred Russel Wallace. Wallace was at the time at the Malay archipelago, studying new species and their variability. He sent Darwin a manuscript for review before sending it to the Linnean Society of London. The paper treated on "the tendency of varieties to depart indefinitely  from the original type". He had reached very similar conclusions to Darwin's about the mechanisms of evolution. 

In the Science trade, priority for a discovery or theory is established following the criterion of the first public presentation. Thus, Wallace's communication to Darwin was a bomb, since it anticipated the conclusions of the laborious work that Darwin had been conducting for twenty years, and that he was in the process of writing down. 
 (footnote 1). Under these circumstances and following the advice of Lyell and Hooker, both members of the Linnean Society, Darwin and Wallace agreed to present simultaneously their results during the annual meeting of the Society, that took place in July, 1858, publishing their communications the following year. (footnote 2) After several more months of strenuous efforts Darwin's book on "The Origin of Species" finally got printed on November, 24th, 1859. The first edition of 1250 copies was sold out the very first day; second edition of 5000 lasted a few more days. 
 

[footnote 1]   This was a very notorious case (though by no means unique) of independent scientific works published simultaneously. Cases like that are even more common nowadays due to the increase of the scientific community and to the quicker and wider divulgation of scientific results.

[footnote 2]  About priority in the theory of evolution through natural selection both scientists were very considerate of each other. Darwin did not feel it was fair for him to step over and rush a publication just because Wallace was about to do it. And on the other hand, Wallace himself would give Darwin most of the credit for the theory all his life. In a 1887 letter to A. Newton he wrote: "I can truly say now, as I said many years ago, that I am glad it was so [that Darwin had so long anticipated him]; for I have not the love of work, experiment and detail that was so pre-eminent in Darwin, and without which anything I could have written would never have convinced the world."

 

Our species found in the victorian English society of the 19th century the adequate conditions for this discovery. The long ruling of Queen Victoria (1831-1901) corresponded to a period of maximum prosperity for the British empire. It begun with the promulgation of commercial freedom; in 1836 the British colony of Australia was created and in 1840 New Zealand also became a colony. Voyages like that of the Beagle were the rule and not the exception. Those were adventures moved by exploration and commercial winds, not by war.(footnote)