ABSTRACT
Chromosomes can determine evolutionary discontinuities which are not morphologically visible. Two geographically separated varieties of Hordeum sativutn give a vast array of segregation in their progeny which is not seen when parents with similar differences come from the same region. The technique of constructing 'chromosome maps' from linkage studies may also provide information about genotypic differences between species, but is open to two serious objections—first, that the amount of research involved in constructing such maps is enormous, and second, that it is often very difficult to establish which factors are truly homologous between different species. Since the discovery of the importance of chromosomes as bearers of heredity, there has been a widespread expectation that the study of the visible structure of these organelles would reveal evidence of outstanding importance for phylogeny and natural classification. Hybridisation experiments have produced systematically interesting results in many groups, in both animals and plants.
