114. VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
must be adapted to growth in close associations. Thus the habitations of Partule 
in the Mariana Islands as elsewhere are determined directly by the occurrence 
of suitable high vegetation of greater or lesser density, and indirectly by the 
physiographic and climatic circumstances which condition the distribution of the 
available areas of vegetation. ‘There are no discernible faunal elements which 
affect the research organisms. 
RESUME OF CHAPTER II. 
Field-work was carried on in Guam during the greater part of July and August 
of 1920, and a brief visit was made to Saipan in the last week of July. Nearly 
10,000 snails were collected, and from the viviparous adults an abundance of 
embryonic material was dissected for use in connection with certain problems of 
heredity. The representative collections were taken from 39 localities of Guam 
and from 8 of Saipan. 
Guam is inhabited by 4 species, 3 of which are not known to occur elsewhere. 
Partula salifana is a new discovery, and was found only on the remote and isolated 
peak of Mount Salifan, from which it is named. ‘The old species called P. fragilis 
by Férussac was rediscovered in small numbers and in restricted areas; it has re- 
mained virtually “lost” for a full century. Partula radiolata Pfeiffer is abundant 
and widespread, and its intrinsic diversification has been carried far. The last 
species, P. gibba Férussac, provides the largest number and the greatest variety 
of subordinate forms. Saipan is populated only by P. gibba, which exists in con- 
siderable number; the material obtained in that place is important on its own 
merits and also because it is available for an inter-island comparison with the 
members of the identical species of Guam. 
The methods of analysis are the same which were employed in the study of the . 
species of Tahiti. The standard characters of the shells were measured and the 
statistical constants were determined according to the accepted biometric pro- 
cedure, in order to obtain precise quantitative descriptions of the local groups and 
also to discover real differences which would have escaped observation otherwise. 
In detail, the investigation is concerned with the qualitative and quantitative 
characters of the shells, with the variations of these characters, and with the dis- 
tribution of the species and of their varieties, always with due regard to the ecologi- 
cal conditions and to the problem as to the relative value to be assigned to the con- 
genital and the external factors of organic constitution. 
RESUME OF CHAPTER III. 
The new species, P. salifana, is noteworthy on account of its close restriction 
to the bush near the summit of Mount Salifan. Its numbers are sparse, like those 
of the associated species in the same area. ‘The shell is conventional in form and 
heavy in texture, and in these respects it strongly resembles the prevalent types of 
the Society Islands of remote Polynesia. One very important point is the existence 
of this heavy-shelled species in a region where there is no limestone, because it is 
clearly evident in this case that the character of the soil is not a factor of the degree 
to which the shell thickens. The embryonic young attain their colors early in this 
species. 
