Looking for a scifi novel about a warrior that came to our time and people are sent to bring him back to protect them from some dangerous creatures

Question

I read a book years ago about a future world in which the people were fat and lazy and had trouble with some type of dangerous creatures so they created a warrior class to fight the creatures. They had time travel and attempted to put a block in time so that none of the creatures could get through but one did anyway. They had also blocked the warriors so now they had no one to fight the creatures. They sent a male and female toad people team back to the 20th or 21st century to get one of the warriors that had gone back and was working as a teacher at a college making videos for use in the classes. One of the things I remember was the man and woman telling him if they tried to save time by learning things then leaving the information for themselves to find later that would "pinch out the loop" and the information would not be there.

Anyone know the name of this book? Thanks.

Answer 1

I'm pretty sure that this is Beyond the Barrier by Damon Knight. Also known as The Tree of Time.

Your plot summary is a dead ringer. Teacher is taken into the future by "Uglies" to kill the last Zug.

A couple of links:

Wikipedia article

isfdb page

The book reads like A E van Vogt. There is a suggestion that Knight wrote in this style on purpose (I think he was critical of van Vogt)