When darwin wrote on the origin of species he assumed due to the fossil record that species would not evolve quickly since that time evolutionary theorists have documented example of much more rapid evolutionary change now how would we explain the long periods relative consistency with little change in some species that we see in the fossil record?

If there is no reason to change (constant climate), then change won't occur. For example, a desert is basically always dry and hot with sand and few clouds. If a creature is fit for the climate, it will continue to exist until a severe enough climate change. For a real world example, in pre-industrial Europe, the birch trees were white. This permitted white moths to survive and not get eaten by the owls that hunt them, until the industrial revolution. Ash got on the birch trees, changing the color of the trees to a darker black. The white moths stood out, and therefore got eaten, permitting the black variant of the moths to thrive, where they were close to complete eradication before the revolution.