For a mutation to have an affect on the species as a whole, it must: Be neutral and not affect the survival of the individual
Be inheritable and be able to be passed on to offspring
Be able to benefit the individual

The answer to this will be from your lab notes/procedure. it would be best to ask another student who participated in the lab as we do not know what was done, what enzyme or if the temperature is celsius or fahrenheit. if i were to guess though i'd say the rate stopped due to enzyme denturing at high temperature