116 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
as factors of the structural variations of the shells belonging to different color- 
groups. | es 
A seasonal cycle of reproductive activity is indicated by the data of fecundity. 
With the advent of hotter weather and of heavier rains in late spring, the animals 
enter a breeding period which apparently continues throughout the calendar sum- 
mer. The data of heredity prove that the several color-classes intercross, and 
that they are not so isolated physiologically as to be real taxonomic varieties. 
RESUME OF CHAPTER VI. 
Partula gibba Férussac is designated by Pilsbry as the most typical among the 
species which occur in the Mariana Islands. It is the most abundant species in 
Guam and is the only one known to exist in other islands of the group. The phe- 
nomena of its geographical diversification are the same as in the case of radiolata, 
and the conclusion drawn from such data is identical in principle. The animals 
occur nearly everywhere in the suitable areas of vegetation in Guam; their vary- 
ing numbers and the statistical characters of the shells of the several colonies are 
independent of the environment, so far as observation goes. 
This species surpasses radiolata in the range of its variations in color. Seven 
primary color-classes are recognized, and in four of these there are distinguishable ~ 
color-orders. Thése groups, duly named on the basis of their distinctive colors, are 
not accorded the status of taxonomic varieties, because their differences in color 
are not deemed sufficient to warrant such a course. Colonies differ greatly among 
themselves as regards the number of color groups they comprise and also as regards 
the relative number within their constituent classes and orders. Furthermore, the 
several color-groups differ in their total numerical abundance in Guam and in the 
extent of the territories throughout which they range; some are virtually ubiquitous, 
while another may be found in a single locality only. The statistical characters of 
the color-groups, like the qualities of coloration, are not influenced by the sur- 
rounding ecological conditions. 
A seasonal reproductive cycle is not clearly marked in the case of gibba, but 
probably the animals are periodic in their breeding habits. While the data of 
heredity are not so demonstrative on account of the slow development of the defin- 
itive modes of coloration in this species, yet there are many considerations which 
prove that the several color-classes interbreed, as in the case of radiolata. 
RESUME OF CHAPTER VII. 
The representatives of P. gibba which were secured in Saipan vary colonially 
in their statistical characters and in their qualities of color. Four of the Guam 
color-groups reappear in Saipan and constitute the bulk of the Partula population; 
the remaining snails are placed within the limits of another of the Guam classes, 
but they display a different ordinal mode of coloration. The gibba from Saipan are 
collectively different from those of Guam in their statistical characters, although the 
degrees of difference are not very large. In brief, the inter-island comparison of 
the gibba representatives discloses a differentiation in accordance with the separa- 
tion of the two localities, manifested by the existence of certain endemic color-forms 
in each place and by the distinctions in quantitative respects. 
