How was the policy of paternalism like social darwinism?

Assuming you mean "social Darwinism," there is no similarity. Social Darwinism postulates that those members of society who have the most social value will prosper and that social failures will breed less (have fewer or no children).

Paternalism postulates that it is morally better to help social unfortunates, including extending their lives and breeding opportunities (help them to have more children who survive and perpetuate their gene lines).

hope this helps :)