By hybrid vigour we mean the possession by outbred organisms of a number of characters which would confer fitness in a wide range of environments. This definition deliberately includes both cases where heterozygous individuals are fitter in a Darwinian sense in the wild, and cases of species or other distant hybrids showing vigour in, for example, efficient utilization of food, long life, or high resistance to disease, although of low fitness because of infertility or for some other reason. Both types of phenomenon are well known, but whether it is convenient to consider them under the same heading will depend on the aspect of the problem being studied. For example, Dobzhansky (1950) preferred to divide hybrid vigour into ‘euheterosis’ as found in wild populations, and ‘luxuriance’ as exemplified in species hybrids. Such a distinction is a helpful one when the aspect studied is the role of natural selection in maintaining a balanced polymorphism in the wild. However, the high level of heterozygosity maintained by natural selection in many wild populations suggests that for many pairs of alleles the heterozygote is fitter than either homozygote. Where we have some knowledge of the mode of action of the different alleles present in a wild population at a given locus, as we have, for example, for the blood groups in man, it appears that different alleles produc qualitatively different substances. This has led to the suggestion that outbred organisms may be biochemically more versatile (Haldane 1948, 1954; Robertson & Reeve 1952). It is such versatility that all ‘hybrids’ may have in common, whether they result from outbreeding in the wild or from distant crosses made in the laboratory.