If the light colored moths were better adapted to the light colored trunks of the trees. Why were dark colored moths still present in the population of Biston betularía prior to 1845

The answer to the question why were dark colored moths still present in the population of biston betularia prior to 1845 in spite of having the light colored moths are better adapted to the light colored trunks of the trees has a lot of reasons why. Maybe, the transfer of the generation had an impact to them.