Page:Land Mollusca of North America (north of Mexico) Vol. I Part 1 i-276.pdf/15

Rh bounded, both as to characters and geographic relations, although typically quite distinct, like the races of Mesodon appressus.

3. There is a third racial category, the "forms" of the present work. My first intention was to use this term only for ecologic forms, those whose peculiarities are presumably owing to the action of some special factors of their station on a genotypically unchanged stock; as in the case of colonies of constantly small individuals of species normally larger (as in forms of Oreohelix concentrata, O. strigosa, Stenotrema hirsuta and many others), or colonies of thin shells from humid forest, deficient in lime, of species which arc normally thicker in dry limestone districts. However, it must be admitted that in a part of such cases the inference as to genotypic constitution of the forms is merely presumptive, in the absence of experimental data.

The term forma has also been used for forms which show some differentiation, consistent in the colony, but either below the grade usually associated with subspecies, or restricted to single or few colonies, thus having a much narrower range than is usually covered by a subspecies, and without noticeable difference in the local conditions. "Forms" or "little races" of this kind are numerous in Oreohelix. They seem to be equivalent to the "microgeographic species" of Dobzhansky.

4. Variable snail populations appear usually to be mixtures composed of individuals of different genetic composition existing in intergenerant colonies. They are often readily separable into two or several lots according to their phenotypic characters. Examples arc seen in most colonies of Mesodon thyroidus, which contain examples with and those without a parietal tooth, and colonies of Cepolis or Oreohelix containing both banded and plain shells, or Oreohelix jugalis vortex, where the lots are divisible into banded and flammulate shells. If there is no advantage in either condition, the mixed population will continue indefinitely.

Most of the variation within colonies appears to be of this nature. Mutations which appear tend to spread, when not disadvantageous or lethal; and they may affect any part of the organism, such as count of teeth of the radula, or of ribs of the jaw, as well as the shell. These several categories below the species have been all treated by some authors as subspecies, and in our rigid system of nomenclature there is no other provision for them, but it is lumping different phenomena under one term.

The number of recognizable subspecies and "forms" in our fauna is very great. In the text following, their characters have been given in all cases where names have been applied, even when the distinctions appear trivial or ill founded, in order that the reader may have before him all data possibly bearing on racial nomenclature. Moreover, I have tried