\l DIFFICULTIES AND OBJECTIONS 147 



new species. This was the keynote of Mr. Vernon Wollaston's 

 essay on "Variation of Species," published in 1856, and it is 

 adopted by the Iter. J. G. Gulick in his paper on " Diversity 

 of Evolution under one Set of External Conditions " (Journ. 

 Linn. Soc. ZooL, vol. xi. p. 496). The idea seems to be 

 that there is an inherent tendency to variation in certain 

 divergent lines, and that when one portion of a species is 

 isolated, even though under identical conditions, that tendency 

 sets up a divergence which carries that portion farther and 

 farther away from the original species. This view is held to 

 be supported by the case of the land shells of the Sandwich 

 Islands, which certainly present some very remarkable 

 phenomena. In this comparatively small area there are 

 about 300 species of land shells, almost all of which belong 

 to one family (or sub- family), the Achatinellidae, found 

 nowhere else in the world. The interesting point is the 

 extreme restriction of the species and varieties. The 

 average range of each species is only five or six miles, 

 while some are restricted to but one or two square miles, 

 and only a very few range over a whole island. The forest 

 region that extends over one of the mountain -ranges of the 

 island of Oahu, is about forty miles in length and five or six 

 miles in breadth ; and this small territory furnishes about 

 175 species, represented by 700 or 800 varieties. Mr. 

 Gulick states, that the vegetation of the different valleys 

 on the same side of this range is much the same, yet each 

 has a molluscan fauna differing in some degree from that 

 of any other. " AVe frequently find a genus represented 

 in several successive valleys by allied species, sometimes 

 feeding on the same, sometimes on different plants. In 

 every such case the valleys that are nearest to each other 

 furnish the most nearly allied forms ; and a full set of the 

 varieties of each species presents a minute gradation of forms 

 between the more divergent types found in the more Avidely 

 separated localities." He urges, that these constant differences 

 cannot be attributed to natural selection, because they occur 

 in different valleys on the same side of the mountain, where 

 food, climate, and enemies are the same ; and also, because 

 there is no greater difference in passing from the rainy to the 

 dry side of the mountains than in passing from one valley to 



