298 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
separated varieties, such as affinis and rubescens, it is more reasonable to view the 
antithetic or sharply contrasted conditions of the latter as derived from a group of 
snails with a composite make-up like that of the Fautaua colony. Purely sinistral 
rubescens and predominately dextral afinis do not interbreed in the areas that they 
now share in common, while furthermore they could scarcely merge their diverse 
modes of coil as readily as each could arise from a mixed colony, where sinistrality 
and dextrality are almost matters of indifference, where adults of one kind so fre- 
quently produce offspring of the other, as in Pantene at the present time. The 
variations in color and size exhibited by the snails of Fautaua are most reasonably 
interpreted as persistent counterparts of the original varied complex of characters 
possessed by the forerunners of the now divergent and well-demarcated varieties; 
they repeat the points of departure of the lines of differentiation which culminate, 
inter alia, in the small, brown dextral shells of afinis and the large sinistral shells of 
rubescens with red and yellow colors. 
IV. Each primary variety assumed its more or less divergent distinctive 
qualities in a somewhat restricted locality, and not in all parts of the island. In the 
author’s view, the colony of Fautaua Valley is to be regarded as relatively unchanged 
and as a persistent representative in situ of the ancestral complex. The amabilis 
associations, differing among themselves, have collectively departed to appreciable 
degrees from their forerunners without question in the same areas where they are 
now found; the intervening barriers between contiguous habitable areas are such 
as to prevent intercommunication almost if not quite completely. P. rubescens and 
affinis present a very interesting problem, because their ranges are almost coincident 
throughout and because they are mutually antithetic in their distinctive qualities, 
about which enough has already been said; that both varieties originated in the 
eastern sector is indicated by the topographical and ecological conditions of their 
territories, and it is relatively unimportant whether both arose along diverging lines 
in the same sector or different subsidiary regions of the east. The sinistral series 
of the south and west must have developed its characteristics in the south or 
southwest, thence to spread and to differentiate into the three components of to-day. 
Finally /ignaria is undoubtedly a local product of the pro-otaheitana stock that 
became cut off in the valley of Tipaerui. 
From what may be learned about the most recent episodes in this whole history, 
progress in the differentiation of a particular colony is made by the dropping out of 
a character, just as rubescens has lost the dextral form of coil attributed to its 
ancestor and as it has gained the clearer yellow and red colors by the elimination of 
the brown, and also by the process of mutation in the narrower sense. Sporadic 
giants among small-shelled snails and anomalous banded color-forms are illustrative 
of the latter. 
In all of this history, due recognition must be given to the flow and ebb of tides 
of migration, necessarily of slow movement, such as the wave of nodosa that has 
reached beyond Punaruu Valley as far as Orofere. Subsequent processes of differ- 
entiation may or not take place; rubescens is certainly invariable in color-character 
throughout its wide range, while crassa has greatly diversified in its compact sector 
