ISOLATION; OR LAND SHELLS ON HAWAII 87 



1. Because the snails were blown along or moved along 

 in short stages. 



2. Because each colony was permanently separated from 

 all the other colonies, and because each had its own 

 average. 



Let colony after colony be started in this way, let time go 

 on for unnumbered snail generations, and we should expect 

 to find precisely what we do find a series of colonies sepa- 

 rated from each other by short steps of difference. Then, 

 too, we should expect to find that the nearer they are to 

 each other, the more alike they will be ; the farther apart, the 

 more unlike. This also is what has happened to snail shells 

 on Hawaii. 



Dr. Gulick brings these facts out and says that any sepa- 

 ration which prevents one colony from mating with another 

 colony is rightly called isolation. He speaks of geographic 

 isolation, when snails in one valley or on one tree have no 

 chance to mate with snails in another valley or on another 

 tree, or when snails that live under stones never meet those 

 that live on tree tops. He speaks of food isolation, when dif- 

 ferent groups live on different kinds of food ; and he assures 

 us that anything which keeps colonies permanently apart, so 

 that mating is impossible, means isolation for them. He 

 also says that genuine isolation of this sort results at last in 

 a new species. 



This was Dr. Gulick's discovery. By means of it he added 

 the law of isolation to Darwin's five-linked chain, and by 

 doing this he made the chain itself so much the stronger. 



Human beings as well as snails, plants as well as animals, 

 are controlled by the same law of isolation. The next chap- 

 ter will show that colonies which mix freely with each other 

 have the smallest number of species. 



