This is unsupported because although climate changes are sufficient to produce migration, they may not be the only condition sufficient to produce migration.

This is unsupported because while shifts in the climate will lead to an intermingling of ideas, we don’t know if these are net positive for human progress. Also, these ideas are necessary but not sufficient for advances in civilization.

This is strongly supported because the author states that large changes in the climate always result in migrations. The contrapositive says that if there are no migrations (the population is settled) then the climate is not largely changing (the climate is fairly stable).

This is unsupported because it is possible that human industry can be established in places where there is an intermingling of ideas as a result of migration.

This is unsupported because while we know that migrations lead to intermingling of ideas, the intermingling of ideas is a necessary, not sufficient, condition for bringing about advances in civilization.