ABSTRACT
In the normal absence of a genetical breeding barrier, the maintenance of the distinctness of subspecies in nature must result from a relative rarity of interbreeding between them, and perhaps also at times from the less 'adapted' character of the products of such interbreeding. Natural subspecies are, it is generally agreed, potentially at least, a stage in the development of full specific difference. Most biological systematists will admit that, in some cases, objective natural units exist which correspond to the concept of subspecies, and hence are worthy of nomenclatorial recognition. The palaeontologists are liable to encounter special difficulties in classification at and below the species level, particularly when they are dealing with a fairly continuous record of a group of organisms over a period of time of the order of a million years or more, as in the so-called lineages.
