instruction:
Combine the given two facts to write a concluding fact. Note that there should be some parts of the first and second facts that are not mentioned in this conclusion fact. Your combined fact should be the result of a chain between the two facts. Chains form when two facts connect together to produce a concluding fact. An example of a chain is: "pesticides cause pollution" (fact1) + "pollution can harm animals" (fact2) → "pesticides can harm animals" (conclusion fact). Parts of the concluding fact overlap with the first and the second fact. To construct such a  concluding fact, a useful way is to borrow the subject from one fact and the ending conclusions from another fact.
question:
Fact 1: a valley is formed by a river flowing. 
Fact 2: Rivers erode the rocks they flow over.
answer:
Water erodes rocks as they flow over, creating a valley.


question:
Fact 1: webbed feet are used for moving faster through water by aquatic animals. 
Fact 2: Aquatic animals moving through water are also subject to the principles of aerodynamics.
answer:
Aquatic animals in water use webbed feet due to the principals of aerodynamics.


question:
Fact 1: Mutualism is a symbiotic relationship in which both species benefit. 
Fact 2: Out of the 8,400,000 different species of living beings, there are 400,000 human species.
answer:

Mutualism is a symbiotic relationship in which both living beings benefit.