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 sinis- 

 trorsa is found. This species, 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. Of its individuals 90 per cent are sinistral and 10 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 Partulse in the Tahitian valleys are 

 isolated very much as are the Achatinellida; 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 snails in 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 Comparative Zoology 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 oj 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 



