370 



ANIMAL BIOLOGY 



geographical barriers. We know, for example, that a litter of 

 European Rabbits was introduced on the small island of Porto 

 Santo during the fifteenth century and by the middle of the last 

 century its descendants had become so distinct from the parent 

 form that they were described as a new species. (Fig. 92.) 



Fig. 237. — Successive forms of a Snail, Paludina, from the Tertiary deposits 



of Slavonia. (From Lull, after Neumayr.) 



As a matter of fact the characteristic fauna of islands was what 

 impressed Darwin with the need of some interpretation of their 

 origin other than by special creation. During his famous three 

 years' voyage around the world on the "Beagle, " he stopped at the 

 Galapagos Islands, situated about 600 miles off the west coast of 

 South America, and was astonished to find that although the fauna 

 as a whole resembled fairly closely that of the mainland, neverthe- 

 less the species for the most part not only were different, but even 

 those of the separate islands were distinct — the islands nearest to 



