How does darwin's theory differ from that of wallace?
They both came to similar conclusions about how species developed.
They both presented ideas that environmental pressures acting on species lead to some surviving and producing more offspring than other members of the species so that the characteristics that allowed them to survive become more common in the population.
This was just a story of who published first, Wallace and Darwin had a correspondence and discussed theories very similar to what was presented in 'On the origin of the species'.
You can read an article on the history here: http://evolution.berkeley.edu/evolibrary/article/history_14