82 GROUPS OF ALLIED SPECIES [Chap. X. 



so freely open from south to north as they are at 

 present. Even at this day, if the Malay Archipelago 

 were converted into land, the tropical parts of the 

 Indian Ocean would form a large and perfectly enclosed 

 basin, in which any great group of marine animals 

 might be multiplied ; and here they would remain 

 confined, until some of the species became adapted to 

 a cooler climate, and were enabled to double the 

 Southern capes of Africa or Australia, and thus reach 

 other and distant seas. 



From these considerations, from our ignorance of the 

 geology of other countries beyond the confines of 

 Europe and the United States, and from the revolution 

 in our pakeontological knowledge effected by the dis- 

 coveries of the last dozen years, it seems to me to be 

 about as rash to dogmatize on the succession of organic 

 forms throughout the world, as it would be for a natura- 

 list to land for five minutes on a barren point in 

 Australia, and then to discuss the number and range cf 

 its productions. 



On iht Appearance of Groups of allied Species 



FossUift rous Strata. 



There is another and allied difficulty, which is much 

 more serious. I allude to the manner in which species 

 aging to several of the main divisions of the animal 

 kingdom suddenly appear in the lowest known fossili- 

 ferous rocks. Most of the arguments which have 

 convinced me that all the existing species of the same 

 group are descended from a single progenitor, apply 

 with equal force to the earliest known species. For 

 instance, it cannot be doubted that all the Cambrian 



