Hi
It is true that on the Galapagos islands Darwin observed tortoises with a dome and saddle shaped shells, you can see both in attached image.
Darwin observed that Galapagos Tortoises had the shells with different shapes depending on the Island they live, For example: The tortoises with saddle shaped shells were able to reach up to eat leaves of plants while the tortoises with dome shaped shells would not reach up and they would eat food from on ground sources. This made him think that the differences in tortoise shell is also a strong evidence that indicates that how tortoises have adapted themselves to better get their food as per the available resources and how they have evolved from same ancestral specie into two different species. This point also helped him in proposing the theory of evolution by natural selection.
Hope it helps!
