GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. A gi 



nous group. For although both have changed with time, 

 yet the African, more than the Malagasian, because of 

 the struggle which the former alone suffered. 



5. firitish Isles. — The fauna of the British Isles is 

 substantially the same as that of the European conti- 

 nent, but there are some very significant differences. 

 (1) It is f a.x poorer in species, and this is especially true 

 of Ireland. For example, of mammals — Europe has 

 ninety species, England forty, and Ireland only twenty- 

 two. Of reptiles and amphibians Europe has twenty- 

 two species, England thirteen, and Ireland four. (2) In 

 many cases there is a difference, but not sufficient to 

 make species; the differences are varietal instead of 

 specific. Such are the facts. 



Explanation. — In preglacial times the British Isles 

 were a part of the continent and inhabited by the same 

 animals. In glacial times they were covered by the ice 

 sheet and all animals were destroyed or driven south- 

 ward. After glacial times and the withdrawal of the 

 ice sheet they became again a part of the continent and 

 were recolonized from Europe. But the time of con- 

 nection with Europe after the withdrawal of the ice 

 sheet was too short for complete colonization, especially 

 of extreme Ireland. Some species had not colonized at 

 all and more had not yet reached Ireland, when by sub- 

 sidence the islands were cut off from the continent, and 

 at the same time Ireland from England. This entirely 

 explains the comparative poverty of the fauna. Now 

 the slight differences. The time has been too short and 

 isolation too imperfect for great divergence. Diver- 

 gence has commenced, but has gone only to varietal and 

 not to specific differences. 



6. California Coast Islands. — Heretofore I have 

 spoken almost wholly of faunas. In this case I will 

 deal with the flora. But the principles are the same. 



