Why did Darwin do what he did?

Answer 1

Assuming you mean become a naturalist; he had a difficult time choosing a career - originally he trained as a doctor (his father was a doctor), but was too squeamish for the operations (which at that time were done without anaesthetic).

The next plan was for him to become a Rector in the church of England; at that time, it was common for "failed" doctors to go into the church. During his theology training at Cambridge, he became deeply interested in natural history - collecting beetles was a hobby.

So, as is quite common, Darwin discovered the subject he was studying at university wasn't really what he wanted to do and he jumped at the opportunity to travel on the Beagle (originally as the captain's companion, he later became the ship's naturalist).

As it turned out, Darwin was an extremely gifted observer, he'd also read some (for the time) revolutionary geology and sociological books which made him think about processes taking a long time. For example, he discovered fossil sea shells in the Andes, he also experienced an earthquake in South America that raised the land by a few metres. He realised that if there were thousands of earthquakes over millions of years, mountains could be raised.

A visit to the Galapagos Islands was also instructive for Darwin; he realised that:

The islands were formed relatively recently - and initially without life.

The life that had arrived had either flown there or "floated" there on vegetation rafts.

Back in the UK, Darwin wrote up his experiences and became one of the most celebrated naturalists in the country. He began thinking about what would happen to species over long periods of time. As a "country" gentleman, he knew how farmers used artificial selection to develop breeds and varieties and realised that because in nature, far more offspring are produced than actually survive to breed, the offspring that have characteristics that make them more likely to survive will be passed on to the next generation.

Darwin was aware that he wasn't a qualified naturalist, and to gain scientific credibility he spent years researching whelks (he became a world authority).

He was still reticent to publish his theory on natural selection; he spent many years gathering evidence to put it on a strong footing. It was receiving a letter from another naturalist who had independently developed the same theory that prompted Darwin to "rush" the book into print.