Page:American Seashells (1954).djvu/90

72 series of populations on the west coast of Florida which seem to be attempting a “break-away” from the typical form, and to this geographical race the name Melongena corona perspectiva has been given. Perhaps in another million years, through fortuitous isolation (geographical or reproductive) and selection, it will merit recognition as a full species. Elsewhere throughout the range of corona, we find minor groups of variants, some that are individuals stunted by ecological conditions, others that are minor genetic variations which seem to crop up at random in all parts of Florida. These ecotypes, aberrations and varieties, although actors in the evolution game, do not warrant subspecific names.

There have been many attempts to define a species. A very excellent summary of the various definitions has been published in Ernst Mayr’s interesting book entitled Systematics and the Origin of Species (Columbia University Press, 1942). Mayr defines species as groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups by geographical, physiological or ecological barriers. Unfortunately, this biological concept of species cannot as yet be used extensively in the field of mollusks, for malacology is largely in the purely descriptive and cataloging stages, and the majority of species being described today are still based on the old-fashioned morphological species concept.

While the species is considered by some people as an objective entity in nature, nearly everyone agrees that a genus is merely a convenient and arbitrary grouping of closely related species. This is also true of many higher categories such as the subfamily and family which are merely convenient groupings of closely related genera. However artificial, the system is extremely useful, for it permits us to arrange the species in our collections and our scientific reports in a logical, evolutionary and biological sequence.

These are the many morphological features exhibited in mollusks which are used for identifying species and in understanding the evolutionary relationships existing between members of the higher categories, such as genera, families or orders. It must be realized that in some groups of shells certain types of characters, such as number of spines, shape of aperture or color markings are used to distinguish species, while in other groups these will prove useless and reliance may have to be put on the number of folds in the columella, the number of teeth in the aperture or the sculpturing on the operculum. These key features are pointed out in their appropriate places throughout this book.

The verbal tools which are used in the study of mollusks are especially designed to assure a method as accurate as possible for telling apart the