156 SUMMARY AND CONCLUSION. 
On the south side of the island, about 27 miles from the three 
valleys just mentioned, is the valley of Vaihiria, where Partula sints- 
trorsa isfound. Thisspecies, though closely related to the two species 
last mentioned, has adopted a different habit of feeding. ‘‘The 
Caladium and the wild plantain grow here in abundance, but most of 
the snails were found upon the wild turmeric, almost none being dis- 
covered upon the Caladium, and but few upon the leaves of the wild 
plantain.’”’ It is very variable in color, but easily distinguished from 
those of similar color in the valleys first mentioned by its lack of a 
tooth on the body whorl, by its relatively thin and fragile lip, by its 
more constricted suture, and by the lack of variation in the color of 
the young. Ofits individuals 90 per cent are sinistral and ro per cent 
dextral. 
From these facts Doctor Mayer draws the following conclusions: 
Partula hyalina is very stable in all of the valleys, and gives rise to no varieties. 
All the other species, however, are remarkably variable, and give rise to numerous 
color-sports. These color-sports tend to breed true to themselves, and, therefore, 
to originate new color-forms and finally new species. This tendency is, however, 
held in check by frequent inter-crossing with the parent stock, and becomes 
effective only when the new color variety is isolated, or when it displays a remark- 
ably strong tendency to breed true. * * * It is probable that geographical 
isolation plays a most important part in the formation of new species. If two 
valleys be adjacent, their snails are closely related each to each, whereas the wider 
the separation between any two valleys the more distant the relationship between 
their snails. The ridges between the valleys, being either barren or covered with 
vegetation unsuitable to the snails, afford barriers over which the animals must 
find it more or less difficult to pass. Thus the Partule in the Tahitian valleys are 
isolated very much as are the Achatinellide of Oahu in the Hawaiian Islands. 
* * * As far as the very limited observation of the writer goes, there appears 
to be no difference in the character of the snailsin different parts of the same valley. 
The difference between any two adjacent valleys is, however, very marked. 
The full statement of these facts and conclusions will be found in 
Memoirs of the Museum of Comparatiye Zodlogy at Harvard College, 
Vol. XXVI, No. 2, published in 1902. 
5. The Power of the Organism to Control its Relations to the Environment 
Increases with the Stage of Evolution Attained. 
We have shown by direct observation that it frequently happens 
that the same species of snails, when distributed in isolated groups in 
districts furnishing the same environment, establishes divergent 
methods of dealing with the environment, and so determines the form 
of selection to which it is subjected in the different districts. More- 
over, this power of the organism to control its relations to the environment 
is found to belong in a higher degree to vertebrate animals, and espe- 
cially to birds and mammals, while immeasurably the highest power 
