If today you can make teaching evolution in public schools a crime, then tomorrow you can make it a crime to teach it in private schools. Then you can ban books and other educational materials that mention evolution. And then you can ban the very word from all discourse. And then the anti-science bigots will have won.

First

Explanation:

When Clarence Darrow extrapolates his worst-case scenario, he notes that banning the teaching of a subject in one level of school could set a precedent to make other things illegal. The situation he suggests shows that, in his mind, a guilty verdict against John Scopes for teaching evolution in his classroom could lead to an erosion of First Amendment rights.

Slippery Slope

Explanation:

Slippery Slope may be defined as a series of actions or event which leads from one result or action to another in an unintended consequences. Thus it leads to a series of action. For example, if we say that event a will happen then it is obvious that another event B will take place accordingly.

In the context, if I make and consider evolution teaching a crime in public schools, then the next thing I will do is that I may make it a crime for the private schools as well. and then further I will ban all the books and study materials that teach evolution. This is know as slippery slope of making and following one event after the other.