ISOLATION; OR LAND SHELLS ON HAWAII 85 



In 1872, twenty years after his shells were gathered, 

 Dr. Gulick wrote out the answers to his questions. 



He said that on the Hawaiian Islands nature had acted 

 like a careful breeder. It had kept certain groups from mating 

 with other groups, even when they lived very near together. 

 And he shows how this was managed, step by step, from 

 the beginning. 



1. After these volcanic islands had been formed, a few 

 snails drifted to them from elsewhere. 



2. These first snails multiplied where they were and, since 

 they were wretched travelers, they stayed in the same place 

 for numberless generations. 



3. In the course of time a few were carried off by birds, 

 or by wind or flood, or on a broken branch, and left in 

 another part of the same valley, or perhaps on the top of a 

 neighboring tree. 



4. These snails stayed where they were dropped ; they 

 multiplied in their new home and had no chance whatever to 

 mate with the parent stock or with any other snails in any 

 other place. For this reason each new group of descendants 

 became slightly different from its own immediate ancestors, 

 and more different yet from all the other ancestors farther 

 back. It became different because it started with a different 

 average. 



To make this last statement plain, imagine seven birds 

 with beaks as long as shown in the illustration on the next 

 page. Also imagine that two of the birds flew off to a new 

 locality. Now notice the difference in the average length of 

 the beaks in the two groups. 



The same law of average holds true with snails. When a 

 few of these are swept away from the original group, the 

 average size or shape of this new, smaller group is sure to 



