ABSTRACT
Human beings have long been impressed by the permanence and distinctness of the individual kinds of living things; the saying 'God made the species, all else is the work of man' expresses an attitude which is still widely held. The criteria which have been used in attempts to define the species may be summarised under five headings: museum criteria, ecological criteria, physiological criterion, genetical criteria and palaeontological criterion. In animals, alternation of generations is rarely if ever a matter of haploid and diploid individuals; except for such special cases as the males of the Hymenoptera, the haploid stage in animals is represented only by the gametes. A general limitation of the ecological criteria is that they are only properly applicable to species living in the same area—a given species may show habit-differences in different parts of its range, in relation for instance to differing climatic conditions, and organisms living in different areas do not have the opportunity of sexual crossing.
