How did Charles Darwin manage to come to his Conclusion?

-After spending a 5 year journey across the World, studying Plants and Animals as he went he managed to note something -That something was that there was Variation in the Same Species, and that the Better off Variations were often meaning they could Survive longer. -He also noted that those Variations could be passed to the Next Generation -'Survival of the Fittest' as because it was only the Strongest who could win it