The Factors of Evolution 255 



lation there have developed in each valley varieties peculiar 

 to it. In some cases a species is restricted to a single valley, 

 while in others it may extend over two or three adjacent 

 ones. The most nearly related forms are found in adjacent 

 valleys, and the most divergent in those widely separated. 

 Gulick, who has made a special study of these snails, says, "I 

 had found not simply a large section of the world, within 

 which peculiar species had originated, but ascending a certain 

 mountain ridge a few miles from Honolulu, and looking down, 

 I could say, 'That valley to the right, a couple of miles in 

 length and half a mile in width, is the birthplace of the 

 Achatinella producta and Achatinella adusta ; and within the 

 groves of this valley upon which we look on our left were 

 created Achatinella stewartii and Achatinella johnsonii; 

 while behind us a mile to the northeast, in the jungle that 

 clings to the almost precipitous cliffs on the other side of the 

 backbone of the island, is the secret home of the very rare 

 and beautiful Achatinella versipellis. ' ' ^ 



Crampton, who has made an extensive study of the distri- 

 bution, variation and evolution of the land snails of the 

 genus Partula, inhabiting Tahiti, one of the Society Islands 

 of the South Pacific, forms closely related to the Achatinel- 

 lidse of Hawaii, has reached conclusions similar to those of 

 Gulick. He finds that "with only one exception each group 

 of islands has its own characteristic species which occur no- 

 where else," while with few exceptions each island in the 

 different groups "possesses distinct species not found in the 

 others," and the species "may vary from valley to valley 

 of an island; one form sometimes extends over a wide range, 

 while another may be restricted to a few valleys or even to 

 one."^ 



In many of the habitats of the different species of snails of 

 Oahu and Tahiti, the environment is seemingly identical. In 

 two adjacent valleys, but two or three miles apart, different 

 species of snails may be found, feeding on the same trees at 

 similar altitudes and experiencing the same degrees of tem- 

 perature, humidity and- barometric pressure. In some cases 

 contiguous valleys present greater differences in vegetation 

 than those more widely separated, and yet the diversity of 

 the snails in the former ease is less than in the latter. Di- 

 vergence of environment is therefore obviously not the cause 

 of the differences involved. Nor is there any apparent in- 

 fluence of selection. When two environments differ widely, 



'Gulick, "Evolution, Racial and Habit udinal," Carnegie Institution, 

 Publication 25, pp. 1-2. 



'Crampton, "Variation, Distribution and Evolution of the Genus 

 Partula," Carnegie Institution, Publication 228, p. 11. 



