CHAPTER VIII. 
SUMMARY. 
RESUME OF INTRODUCTION. 
The investigations described in the present volume are concerned with the 
natural history of the representatives of the genus Partula which dwell in the 
Mariana Islands, Guam and Saipan. An earlier volume on the species of Tahiti 
developed the methods and recorded the results of an intensive study in the 
present headquarters of the genus in southeastern Polynesia; as the Mariana 
Islands lie far in the western Pacific Ocean and quite at the northwestern limit 
of the tropical area in which the genus ranges, the present study provides the 
requisite materials for a comparison which serves to bring into clearer relief such 
principles of ecology, distribution, and relationship as are common to the species 
of widely separated regions. 
RESUME OF CHAPTER I. 
The geographical conditions throughout Oceania are ideal for the study of 
the correlation between the resemblances or differences exhibited by related 
species and the proximity or isolation of their territories; with rare exceptions, 
each group of islands bears its own distinctive species and each island within a 
group possesses its characteristic forms, while different localities of one and the 
same island may be inhabited by diverse species or varieties. The Mariana 
Islands are the dry peaks of a submarine range which extends in the form of an 
upright bow for a distance of over 400 miles; they are remote from other groups 
except to the north, and their present disconnection is emphasized by the great 
depths of the ocean in their vicinity. Their very isolation argues against their 
population by Partula immigrants from another group of islands. 
The northern members of the Mariana Group have been formed by volcanic 
action in relatively recent geologic times, and they are devoid of Partule; the 
older southern elements are composed partly of volcanic materials and partly of 
coral limestone upraised above the sea, and they only are inhabited by the larger 
land-snails. The northern part of Guam is a plain, almast exclusively calcareous, 
in marked contrast with the higher southern region of volcanic mountains and 
slopes where the lower levels only are composed of limestone. Saipan is a unitary 
conical mass of double geological nature, like the southern part of Guam. 
The climate is hot and wet, with a somewhat definite seasonal cycle. As the 
calendar summer approaches, the northeast trade-winds give place to the south- 
west monsoon; accordingly the temperature rises and the rains become more copious 
until early autumn, when the reverse changes occur. The thick growth of the 
forests constitutes the most favorable habitation of Partule, with the requisite 
conditions of shade, moist atmosphere, and humus for food. The thickets about 
the plantations and villages are less suitable, while the open grasslands of the 
savannas can not be tolerated by the snails. The botanical characters of the 
plants among which the animals are found are negligible, saving only that they 
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