To date, no agreement has been reached on the definition of taxonomic categories such as species, genus and family. This means that there are no generally accepted definitions for terms that biologists deal with on a daily basis.

The authors of Basic Types of Life draw attention to the previously little-noticed possibility of characterizing kinship relationships in an experimentally verifiable manner through interspecific crossings. The genetically based Basic Type category based on this is superior to the species category and, in the case of the organisms from the animal and plant kingdoms studied so far, lies between genus and family. Basic Types represent clearly demarcated groups.

Species concepts are usually based on the assumption that speciation is correlated with development from a lower to a higher order. By contrast, the distribution of characteristics within the species ranges of individual Basic Types is interpreted within the framework of the hypothesis of genetically polyvalent ancestral forms from which genetically less flexible descendants have arisen through speciation processes.

Basic Types of Life is a translation of the German classic biological work, Typen des Lebens. This translated volume also incorporates new hybridization and other data that sheds further light on previous studies.