单选题
By examining fossilized beetles, a research team has produced the most detailed description yet of temperatures in Britain over the past 22,000 years. Fossils of species that still exist were selected and dated. When individuals of several species found in the same place were found to date to the same period, the known temperature tolerances of the existing beetle species were used to determine the maximum summer temperature that could have existed at that place and period. The procedure of the researchers assumes which one of the following?