In Darwin's theory, he noted that all species tend to over-reproduce. He mentioned that if they all survived, the population would grow, however, most populations remain stable. They are competing for resources--food, shelter, space, mates, etc. Therefore, there is a struggle for existence. All members in a population have differences called variations. If an individual has a variation that helps it survive, it may live and reproduce and pass this on to offspring. This is called survival of the fittest. Those most suited to the environment are more likely to survive than those who are not. Over time, natural selection occurs and those with the best suited traits will survive and pass those on. This can result in the population inheriting enough changes that a new species will occur. He used Thomas Malthus' work on Populations that argued that if unrestrained, populations breed beyond their means and struggle to survive. He also was influenced by his journey to the Galapagos Islands as a young man where he saw finches on the various islands with different beaks adapted to different diets. He theorized that the closest land mass which was Equador must have provided the ancestral stock and that over time, the birds must have struggled to survive. Those with variations that provided an advantage in their new environment were selected by nature, lived and reproduced. Over time, many new species of finches evolved.