Coolidge here is talking about the Immigration Act of 1924, a bill he signed after earlier presidents had vetoed it.

IMMIGRATION

American institutions rest solely on good citizenship. They were created by people who had a background of self-government. New arrivals should be limited to our capacity to absorb them into the ranks of good citizenship. America must be kept American. For this  purpose, it is necessary to continue a policy of restricted immigration. It would be well to make such immigration of a selective nature with some inspection at the source, and based either on a prior census or upon the record of naturalization. Either method would insure the admission of those with the largest capacity and best intention of becoming citizens. I am convinced that our present economic and social conditions warrant a limitation of those to be admitted. We should find additional safety in a law requiring the immediate registration of all aliens. Those who do not want to be partakers of the American spirit ought not to settle in America.

[Calvin Coolidge: First Annual Message]