DISPERSAL OF ANIMALS 325 



from an ark, and thus a colony of several hundred new species 

 may at once be naturalized." 1 



As a definite example of this kind of dispersal may be men- 

 tioned the fact that in 1827 a large boa constrictor, twisted round 

 the trunk of a tree, was carried by ocean currents from South 

 America to the Island of St. Vincent, where it was destroyed 

 after killing a few sheep. 



A current flows from the North Island of New Zealand 

 southwards to Chatham Island, four hundred miles distant 

 from the nearest point on the New Zealand coast. This current 

 carries considerable quantities of New Zealand timber to the 

 island and its existence probably accounts for the fact that the 

 planarian worm, Geoplana exulans, has been found both in the 

 North Island of New Zealand and on Chatham Island, but, as 

 yet, nowhere else. The land planarians habitually creep into 

 the crevices of decayed timber, and their eggs are enclosed in 

 tough, horny cocoons which may probably occasionally be 

 transported even over wide stretches of sea. 



Small terrestrial animals are, of course, often accidentally 

 dispersed by human agency. Rats, mice and cockroaches have 

 been carried nearly all over the world by ships, and snails, 

 worms and other small creatures may be carried about with 

 timber and earth, especially around the roots of plants. When 

 I was in New Zealand I had some plants sent to me from 

 England in a tightly closed tin box. When they arrived, after 

 a voyage of some five or six weeks, I found an earthworm still 

 alive in th^ tin. Many invertebrates have doubtless been 

 unknowingly dispersed in this manner and great care has to be 

 taken to make due allowance for such possibilities in studying 

 problems of distribution. In exactly the same sort of way the 

 seeds of many plants are accidentally dispersed over the world 

 in ships' ballast, so that the same common European weeds 

 occur in the neighbourhood of the ports along all the great 

 routes of commerce. 



The restrictions placed upon the dispersal of fresh water 

 animals are more severe than in the case of either marine or 

 terrestrial forms. One river system or one lake is separated 

 from another by intervening land or sea which fresh water 

 animals cannot as a rule cross by their own exertions. There 

 are of course exceptions, as in the case of the lampreys and eels, 



1 Lyell's " Principles of Geology," Ed. 5, Vol. III., p. 44. 



