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.

Fact 1: Lancelet larvae are free-swimming. 
Fact 2: Some types of snails are free swimming, while others are sessile.
lancelet larvae are not sessile.

Fact 1: a valley is formed by a river flowing. 
Fact 2: Rivers cause erosion by transporting weathered rock and soil downstream.
Valleys are formed by erosion.

Fact 1: natural disasters can cause animals to leave an environment. 
Fact 2: Environments are animals homes.

natural disasters can cause animals to leave their homes.