PARTULA RADIOLATA. a7 
In the Southwest and Southeast Regions radiolata forms the major and almost 
the exclusive population, owing to the scarcity of gibba. The figures for Inarajan, 
Lower Ylig, and the Pago River are especially interesting because the areas of col- 
lection were the lower levels of the respective valleys, where gibba either does not 
occur at all or is very rarely found. On the higher ground of the Upper Ylig locality, 
Pago North, and across the island at Macajna and Fonte, radiolata is less abundant, 
actually and relatively as well, for gibba thrives plentifully at the latter places. But 
the Dededo region extends at approximately the same level as the Macajna, Fonte, 
and Barrigarda localities. It is not permissible, therefore, to draw the conclusion 
that the present species is generally more abundant at lower than at higher levels; 
hence the barometric factor is also to be discarded as conditioning the numerical 
frequency of the species. 
The character of the soil is likewise a matter of indifference. In the suitable 
areas of vegetation in the Umatac territory, where the ground is volcanic in earlier 
origin, radiolata lives quite as well as in similar situations in the northern half of 
Guam, where the rock is uplifted reef limestone. 
In summary, therefore, we can only say that the numbers of radiolata are 
sometimes many, where conditions seem to be favorable, and sometimes few where 
the circumstances appear to be identical. In any locality which possesses the 
prime requisites of vegetation and moisture, the actual numerical abundance of the 
species appears to depend primarily upon the innate vigor and reproductive rate of 
the organisms. 
COLONIAL VARIATION IN QUANTITATIVE CHARACTERS. 
We now come to the detailed consideration of the variations displayed by the 
representative collections of radiolata secured in the several localities of Guam. 
When the whole field is broadly surveyed, the colonial series differ not only in the 
matter of the density of population, but much more fundamentally in the statistical 
constants that accurately describe the measurable characters of their shells. Such 
differences are quite independent of variations in color composition, in which two 
modes occur, inasmuch as the colonies vary as regards the kinds of color types that 
are present as well as in the relative numbers of the component color-classes. No 
collection from a given locality, ample enough to be truly representative, is exactly 
similar to that from another place; where the comparable series are small, the 
differences are naturally accentuated. The degrees of difference vary, and they are 
sometimes great in one respect, while they are nearly or entirely negligible in 
others; but there are no exceptions to the general rule. Furthermore, no observable 
relation exists between an ecological factor, like the nature of the soil or the num- 
bers of a particular nurse-plant, and the distinctive qualities of a given association, 
whether of size or shape or coloration of the shells. 
The complete description of the shell characters is given in tables 12 to 14, 
which comprise the figures for the individual associations which are sufficiently 
large to warrant the labor of their statistical analysis, for the assembled regional 
populations, and for the whole collection. The extreme ranges of the characters 
