In "The American Species of Crepis", Babcock and Stebbins described the concept of the polyploid complex, and its role in plant evolution. Some genera, such as "Crepis", have a complex of reproductive forms that center on sexually diploid populations that have also given rise to polyploid ones. Babcock and Stebbins also observed that allopolyploid types formed from the hybridization of two different species always have a wider distribution than diploid or autotetraploid species, and proposed that polyploids formed through hybridization have a greater potential to exploit varied environments, because they inherit all traits from both parents. They also showed that hybridization in the polyploid complex could provide a mechanism for genetic exchange between diploid species that were otherwise unable to breed. Their observations offered insight into species formation and knowledge of how all these complex processes could provide information on the history of a genus. This monograph was described by Swedish botanist Åke Gustafsson as the most important work on the formation of species during that period.