150 FREQUENT AND INFREQUENT MIGRATION. [CH. Ill 



dispersal across the sea thinks that this migration is 

 effected involuntarily and in the egg state. As to in- 

 voluntary migration on the part of reptiles Mr Wallace 

 quotes the case of a boa constrictor which floated on a 

 cedar to the island of St Vincent, two hundred miles away 

 from its home, where it killed a sheep before it was itself 

 put an end to. 



Evidence of capacity for Migration on the part of 



a given animal. 



As Prof. Semper has acutely pointed out 1 the facilities 

 for migration possessed by an animal can be to a certain 

 extent measured by the amount of modification gone 

 through by obviously migrated individuals. Naturally 

 no clearly defined scale can be drawn up ; but if an 

 animal, say a Mollusc, can very readily cross the sea to an 

 oceanic island it will be likely that its descendants upon 

 the island will hardly, if at all, differ from the parent 

 stock upon the main land. The reason of course is that 

 migration being easy will be constant and frequent, thus 

 preventing the acquisition of new characters through 

 isolation ; the perpetual interbreeding with newly arrived 

 individuals of the parent race will securely keep the 

 progeny in the original mould. Of course other causes 

 must also come into play, but this fact cannot be without 

 significance. 



It has been already pointed out that Reptiles are rare 



1 Animal Life, Int. Sci. Series. 



