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In the Rocky Mountains beaver dams slowly filter billions of tons of water. The ponds build up soil and nutrients and help prevent floods and _____.
In a _____ model the Sun, not the Earth, is in the center and all other objects including the Earth orbit the Sun. In this model the Earth is moving in two ways. It revolves arround the Sun and it also rotates on an axis.
The beaver's incisors are strengthen with _____ which makes them orange. They grow continuously and even self sharpen.
By about two months old its soft down is replaced with feathers and soon it develops its iconic _____ like crest.
The solar system came into being about 4.5 billion years ago when a cloud of interstellar gas and dust collapsed resulting in a solar _____ , a swirling disk of material that collided to form the solar system.
 On the Origin of Species
How do we explain how animals have evolved over time. Charles Darwin, the British naturalist and geologist, famously answered this question in his book '_____' which proposed the mechanism of evolutionary change within animal species.