Verified answer

I assume you mean his book the On the Origin of Species.

Darwin did a lot of research on the Galapagos Island where he formed the idea that different animals occupy different niches due to natural selection.

There were many different types of finches with each species having slight variations of beaks. These beaks allowed each bird to occupy a different food niche and one bird with particular characteristic that gave it an advantage over other types in eating a particular type of food or exploiting some other environmental quirk. One bird's beak might be better suited to eating hard seeds while another had a beak that was better at eating insects. Over time this led to natural selection (important term btw) since those characteristics were inheritable.